Pointers:
Start out as a rogue to get all the prereq skills you need, evasion, sneak attack dice, and then go over to wizard, picking up alchemy, spellcraft, and knowledge arcana as much as possible take ONE level of assassin to get the death attack and the poision use, then bounce between wizard and rogue depending on your desire to get magic, or get more sneak attack dice and skill points.
keep a high dex, but a higher intelligence - this maximizes your death attack, alchemy skill, and your spell DC's if you have to resort to direct spell assult.
once your dex bonus cracks +8 , there is no real point to normal armor at all, as the "max dex bonus to AC" starts to get restrictive
your base dex bonus should be in the +3 to +4 range, and gloves of dexterity +6 should punch this bonus up 2 to 3 more points. Also - if you are worried about AC - remember that haste gives you a +4 bonus to AC as well - and the goal is a quick kill.
your best bet as far as race is the elf FRCS race that awards a +2 to Dex and a +2 to Int.
as far as spells -
alter self - disguise
invisibility - stealth
sleep - for taking out the guards without too much bloodshed
spider climb - mobility
knock - for beating arcane locks that you can't pick
hold person - which if works could end a fight before it starts shield - defence
mage armor - defence
magic missile - staple
melf's acid arrow - attack spell that you get sneak attack dice on dispel magic - cracking magical defences
fly - mobility
haste - combat mobility
fireball - when you have to go for a bang
polymorph self - disguise, mobility, AND melee punch, in one spell
improved invisibility - shoot from nowhere - get out alive...
your familiar is likely to be an owl ... you will appreciate the skill bonuses... and with the right equipment and skill selection - a whole lot of +2 bonuses crop up in convienent places.
for you - charisma will be of little to no consequence except for the bluff check in combat to throw your opponent off balance and get your sneak attack, and most opponents won't have a bunch of sense motive ranks.
Much beyond 7 levels of wizard you don't have very many goodies in regards to utility spells. Beyond this it is preety much sheer firepower. My best guess is to go ahead and do the following mix:
Rogue 3/Asn 1/Wizard 6
taking one more level of wizard - then reverting to conventional rogue all the way to 20th level
your fort save is ugly - so you probably have to take great fortitude as a feat about 6th level or so
Improved initiative is likely to be your first level feat
As far as the rest of your feats-
craft wondrous item, craft arms and armor, and brew potion are possible "make your own magic item" needs - but beyond that - I don't see much.