A few suggestions:
DO NOT MULTICLASS with any class that does not give you +1 spell level. First off, you are in RttToEE. You need your caster to be as powerful as possible. Currently playing the module right now I can definitely say that any multiclassed casters that focus on their spells/powers are useless in comparison to what our party would have been with just straight casters. This is not to say that multiclassed casters have no place in RttToEE, they just simply ought not to focus on magic. Rather that magic should supplement their fighting or skills. Second you have quite an extensive party. Multiclassing really is not necessary. If RttToEE has taught me anything it is that the only way to survive is to not just have each character be the best that he/she can be but also have every party be the best that it can be. This is not the same as just the sum of each indiviual party member. AKA the whole is greater than the sum of its parts. Take advantage of the synergy between the classes. I will come back to this point later
GO WIZARD!!! (with the following condition). If your DM actually makes it possible for you to use item creation feats, the wizard will wipe the floor with the sorcerer because of the free scribe scrolls feat at 1st level. The XP cost is close to nill for scrolls and you can make them extremely cheaply. If your DM plans on having you enter the module at 4th (the recommended) you will have those 4 levels to scribe as many scrolls as you have money for ahead of time. Thus, your goal should be to learn every spell you can - researching, buying, and defeating other casters. While the sorcerer has more spells per day than a wizard. A wizard with scribe scroll will have more spells than a sorcerer at a very low cost and have much greater diversity of spells. The scrolls only take a day each to make and at higher levels (higher than first) you can for example make a scroll of color spray at caster level 1 and it will be incredibly cheap. If you focus on making spells that you can minimize your caster level you will save a lot of money.
Wizards also incentivize high Int as opposed to Chr with Sorcerers. Int provides bonus skill points while Chr provides no extra bonus at all. Wizards have access to far more PrC's - usually at earlier levels (meaning you can increase your Saves at earlier levels). And wizards get the extra feat(s).
Maximize the synergy between you and the rest of the party. You have 3 warriors a caster and a utility person in your group. Let the cleric worry about buffing everyone and healing everyone. Have the rogue be your scout possibly with the ranger and you ought to take care of all dispelling, counterspelling, and battle control. As an arcane caster you basically get to mediate battles with the use of spells like fireball, wall of force, etc. You will determine the formation of the enemies and who gets to interact with your group (in a fighting sense) and who doesn't. Working with your teammates literally means tactically mediating combat. If faced with a front line of weaker individuals that prevent your fighter types from getting to the casters in the back use color spray, flaming sphere, or some other effect to make a hole in their ranks. If you see a group of powerful creatures that no one can get to, use web. Use invisibility on your rogue to maximize the effect of his sneak attack, etc.
If you still decide to go with sorcerer keep several things in the mind. Along the same lines as was described above with synergy with your teammates, choose spells accordingly.
YOU HAVE A CLERIC IN YOUR PARTY!!!. Do not take any spells that the cleric can cast - they will be an aboslute waste since the cleric could have cast them instead of you.
Do not take any spells that you would not use at any level at any time. For Example, sleep is only useful at low levels - get some scrolls of it - don't even waste money on a wand of it. However, sleep is VERY good at low levels - so definitely do get some scrolls.
Rope Trick - good for resting before you have mass teleport and good if you can outdistance a pursuer. AKA only useful once a day for resting and maybe a few other times besides that - get a wand or a few scrolls.
Change self - useful only until alterself. Both of which are very limited in their applicability - get some scrolls. You could focus more on subterfuge with your sorcerer - in which case the bard multiclass may be a bit more useful as you can get additional languages as well.
Protection from ... (arrows, elements, etc.) only useful a few times - get scrolls.
Knock - get scrolls - just won't use it enough
Jump/Spider Climb/Expiditious Retreat - get scrolls or wands or items
Effective spells to take would be:
Charm Person (if you don't take a wand of it)
True Strike - only if you plan on making potions of it. Simply not useful enough otherwise. Only exception to this is for polymorph, but you can have your cleric buff you to make sure you hit when you polymorph, so still a bit of waste.
Magic Missile - only nonavoidable damage your party will have
Shield - greatest 1st level spell in the game - get bonus to AC, bonus to reflex saves, immune to Magic Missles
Color Spray - incredible at lower levels, still useful at higher levels - stunning for a round is quite nice and will save your party
Web - one of the ultimate utility spells - casts between a flying creatures wings with a readied action for when the wings are parallel, use as a net if falling, create a barrier on the battlefield, immobilize creatures, prevent spellcasters from casting, unless then have the appropriate metamagicked spells, prevent invisible creatures from attacking you while you sleep, etc.
Glitterdust - absolutely requires permanent/semipermanent ability to detect magic or invisibility. However, if you have it, one of the greatest single spells because has the effect of dispelling invisibility and harms the enemy
melf's acid arrow - continuous damage, good against creatures immune to multiple attack forms (read - trolls), good against casters requiring concentration check every round, good for destroying inanimate objects such as doors as most objects do not have acid resistance
Blink - (only if you can cast on others - IDHMBWM - I forget if you can cast on target or if it is self only) - if you can cast on others reduces damage to everyone by 50% - combine that with shield other cast by your cleric - and all your party members are taking 25% of the damage they would normally take which effectively quadruples their hit points. Also, a pure boon for the rogue - constant sneak attacks.
Fireball - goes without saying.
Haste - along with Polymorph, may be the most powerful spell in the game
Vampiric Touch - touch attack spell, one of the only ways you will ever be able to heal yourself
Polymorph - goes without saying - may be the most powerful spell in the game relatively speaking
Summon Monster IV or V, V or VI, IX. While the cleric can cast these spells, he will be limited by alignment. You can cast the spell with any creature - one of the best diversity spells for sorcerers.
Charm Monster (if you don't already have Charm Person)
Dismissal (if going for Sacred Exorcist only)
Wall of Force - absolute must - ultimate utility spell - can be used to partition battle field, invisible so existence unknown, can be used to counter many spells like fireball, lightning bolt, etc. - just ready an action to cast a spell and cast it directly in front of caster, can be used to enter hostile environment, etc.
Telekinesis - ever seen the shuriken smack - but even if your DM rules 0 on that still has so many uses
Spells you definitely want wands of if possible are:
Dispell Magic
Suggestion
Invisibility
Haste - if you did not take it as a spell
Polymorph other
Superior Invisibility - don't take as spell if take blink - redundant. I put blink instead of superior invisibility because enemies can detect you when invisible but nothing but a ghost touch weapon will deal full damage when blinking.
Finally, go for Mage of the Arcane Order - the spell pool will be the most useful ability a sorcerer can ever get - EVER! and only the cost of two feats that you might not choose anyway (spell prerparation and cooperative spell) And spell preparation is useful anyway because now you can metamagic spells without the full round action.
If you go with sorcerer, here are a few other things to consider
Take a level of Paladin - you get your chr mod as a bonus to all saves. Let me say that again - YOU GET YOUR CHR MOD AS A BONUS TO ALL SAVES. Believe me when I tell you that this can save your life. Not to mention you get the increase in HP (esp. if you took this as your 1st level). Also you get prof. in all armor and all martial weapons. You can use mithril light armors and mithril shields to negate arcane spell failure chance and significanlty boost your AC esp. if you enchant those armors and spike then and put defending on the armor and shield spikes. I know I said earlier not to multiclass but this may be the one valid exception as the defensive abilities are simply so great. A second level will give you the fear immunity and the bonus to your teammates which can save all of your lives and the smite ability - not necessarily useful for damage but very useful if you do not want to miss with one of your touch attack spells - like vampiric touch. Also, I think this will give you what you need to successfully multiclass into Sacred Exorcist as a Pal. 2/Sor 8/ Sacred Exorcist. If you do wind up taking a level in Paladin make sure to capitalize on your newfound melee prowess - make sure the cleric buffs you up, definitely take true strike and take Power attack as a feat. This seems to significantly diverge from your original concept however.
Take a level of Bard - you can continue to take ranks of Perform (even if cc) and get all the higher level abilities of the Bard just limited to a few people. You get the low level bard spells, you can use wands of curative magic and you get some good skills and languages. Languages can be combined with alter self to very good effect and skills are useful for qualifying for PrC's
Consider Sacred Exorcist as a PrC - perhaps after your 10 levels in MotAO. Sacred Exorcist will give you the same spell progression as any other PrC with +1 spell level/level but gives you turning - augmented with your high Chr, gives great hit points for an arcane caster - 1d8! hp/level - and gives you a domain power and additional spells. If you think about it, each of those additional spells - 9 in total - is equal to the expenditure of 1 feat not to mention some of those spells are not on the sorcerer's normal spell list. You also gain extra turning and some really cool special abilities - constant consecrate rocks my world especially in the ToEE. You will be borderline as good as a cleric at turning and you can get the same items to boost your power. (If you took a few levels of paladin you can also wear the armor giving you those phat bonuses). Alternatively you can take the feats requiring turning as a prereq - Divine Shield and Divine Might could be quite useful.
Finally consider taking leadership - your chr score will be great for giving you the most powerful cohort possible and can have your NPC's be adepts with cooperative spell increasing the effectiveness of your spells.
Since I have written this much I might as well go all the way. RttToEE is the toughest module I have every played in (almost 10 years of gaming time) EVER. It is literally ridiculous. My advice is to min/max the shiz out of everything and everyone. If you don't, your party will die. There is simply no way around it.
Given your party I would recommend the folowing.
Make sure to get Item Creation feats - you need to have the best and most optimized equipment you can afford. I prefer to take leadership and have my cohort have the item creation feat I am looking for. However, do what you like. I would definitely recommend that between your party and their cohorts that someone get craft magic arms and armor. This way your archer can have healing arrows - greatly increases effectiveness vs. undead and is much more effective than any cleric could ever be for healing as the archer can fire so many more arrows per round vs. casting healing spells. Also making spell storing arrows will increase the range of many spells and again have the same effectiveness of the healilng spells vs. cleric casting. You will also be able to make +1 swords with every freakin enhancement possible - things you certainly will not find. That way your clereic can cast greater magic weapon on them and effectively make the weapons epic at greater levels by having temporary +'s and a whole lot of abilities. You can also make armors that increase a few things. For one you can increase the turning capability of the cleric (and possibly sorcerer/sacred exorcist). You can gain partial immunity to ciritical hits, and can give the ranger and scout bonues to sneaking around.
Craft Woundrous Item - most useful item creation feat in existance - so many useful little toys. To name a few: boots of springing and striding, the Haversack, scarabs, Cloak of Protection, Bracers of Armor, Amulets of Natural Armor, Prayer Beads of Karma, etc. etc. etc.
Forge Ring - RING OF WIZARDRY!!! - better for wizards than sorcerers but still darn useful. If just for this alone, get someone to have the feat. You can give the rogue a ring of blinking - sneaks all the time - giddyup. Ring of Counterspells for the barbarian. Ring of Chameleon Power for the Ranger
Brew Potion - POTIONS OF TRUE STRIKE ANYBODY? ALmost useful enough to get the feat simply for the warriors. Again, though, better if you can get a cohort who has it.
SCRIBE SCROLL - not as useful since as a sorcerer you have a limited list of spells - however if you get a wizard cohort - by all means.
Craft Rod/ Craft Staff - don't take - simply not useful enough
Craft Wand - same as scroll - however not as bad if you take a level of bard as you will be able to use a wand of curing.
Get the Barbarian a suit of Mithral Breast Plate - counts as light armor - so no movement reductions - great AC bonus, low armor check penalty and can enchant to +5. Again, spike all armors with +5 defending spikes to add unnamed +5 bonus to AC. If they are wielding a shield as well you can do the same thing. From magical bonuses alone, +5 armor with +5 defending spikes and +5 shield with +5 defending spikes is a+20 armor bonus not including the actual bonus the shield and armor give you. Get gloves of storing and have your cleric cast the temporary weapon enhancement spells like flaming blade and just store it in stasis to prolong the spell (effectively giving permanent ability - again, same principle as greater magic weapon). Also gives virtual quick draw feat with that weapon alone.
Make sure your ranger has the most powerful animal companion he can get (adventuring separately to do so if necessary) and give the pet all the magical stuff possible.
Have your familiar be invisible at all times and deliver all of your touch spells.
Ok,
I am done, hope all that was helpful
Thank you all so much for all of your hard work. I will try a human Sorcerer. We will start out at first level and hopefully have a lot of low level encounters before we enter the Temple.
