G'day Wellard
Where abouts are you up to? Sounds like you're deep into act I. Are you playing with the AI on or off?
I played a Wizard in my first playthrough but didn't have too many issues with tank power - Khelgar should serve you well providing you look after him and keep him outfitted with the best possible equipment. Some buffs from Elanee and the haste spell when in doubt for the harder battles.
Taking advantage of Qara's firepower is important as well. Rest alot to keep her fully powered as much as possible. My main party from my first playthrough varied according to whichever character was forced at the time, but was mainly spell-based:
Khelgar (Fighter - I didn't bother going for the monk levels)
Neeshka (Rogue)
Qara (Sorceror)
Sand (Wizard)
PC (Wizard - Evocation specialist)
Yep - I loved obliterating things with magic.
Gabrielles is correct though - you'll soon be able to get Casavir the paladin and Bishop the chaotic evil Ranger. Either or both should fill the hole you need, depending on your playstyle.
I'm replaying it at the moment as an evil, crazy crusty dwarven fighter - just hit level 11 and let me tell you, Khelgar and he are a sight to behold tearing things apart with a fiery greataxe each.
Cheers,
CD