You might go with a dwarven defender or an arcane archer, but they're not what I'd choose under the circumstances...... Your melee combatants might need the support though, I dunno. If you want to keep it simple or something, you could play a druid and focus on melee durability (maybe multiclass 1-2 levels as a cleric before the druid levels, in order to get some spells like Divine Favor and Shield of Faith); multiclassing the druid into barbarian for 1 or 2 levels instead could net you Rage and Fast Movement, though, along with a teeny bit more HP.
I'd really suggest that you play a human (or elven) wizard 3/cleric 3/mystic theurge 7, focus on MT advancement until complete, then continue wizard advancement, until W7/C3/MT10; you'll initially cast as a wiz10/clr10, and eventually cast as a wiz17/clr13, giving you access to a bit of 9th-level wizard magic and 7th-level cleric magic.
I'd suggest specializing your wizard class as a conjurer or invoker, possibly an abjurer instead. Give up the enchantment and illusion schools in that case, or perhaps enchantment and necromancy, or illusion and necromancy. Let the shadowbane stalker concern himself with information gathering and sneakiness, you'll handle other utility magic and a bit of combat magic. The stalker and the NPC cleric can handle the party's buffing spell concerns, too. I'd likely suggest your cleric class use the Luck and Protection domains, perhaps Sun or Travel instead. Of course, it all depends on patron deity.
You'll be the main utility and support caster, but also have access to very decent attack and defense spells, so you won't be completely overshadowed by the warmages on offense. Learn some good spells for personal defense, and probably keep one or two Summon Monster IV's prepped in case you need some short-range defense against melee attackers. But mostly focus on utility spells and a decent selection of attack spells. The occasional summoned critter can be conjured to support the paladin and others in melee, so make sure to summon the toughest critter you can, rather than worrying about offensively strong critters. Critters that can heal the paladin in a pinch, or draw attention away from him when he's being pummeled, would be best.
You'll be the group's main healer but not entirely stepping on the toes of the other clerics for that, and be able to raise folks (and eventually resurrect them instead), as well as use Restoration and such. I'd suggest taking the Reach Spell feat or whatever it's called, so you can metamagic-cast touch-range spells from a short distance with a ranged touch attack. Shouldn't be too hard to hit the paladin or warmages that way, while avoiding melee proximity yourself since you'll have weak HP. You'll be able to set up Walls of Stone or Walls of Ice to block yourself off from easy melee reach, while shaping them to give you line-of-sight to the frontline combatants (or you could just cast Levitate to get the much-needed LOS).
