[Aerich]

How far have you gotten in the game and what level are you playing at with how many characters?

There should be plenty of spells of your level once you get to Kuldahar. Orrick's inventory changes after each Chapter, so you can buy third and fourth level spells from him after you've been to Dragon's Eye once. There are all kinds of spells on tables in the Severed Hand, which should hold you until you get to Bandoth in Upper Dorn's. You may not have the greatest choice of spells for every level, but most of the time a SC mage has access to spells of his/her highest castable level with a 6-character party on normal.

A single classed thief is perhaps the worst character in the game, but works well enough when combined with another class; I prefer fighter, to give me a dedicated badass archer with enough HP to scout or sneak attack if necessary. I steer away from SC mages because of low HP and other combat values.

Dual class is fine, especially if you have a bard, as you will not have to spend any thieving points on pickpocket. Be warned that dualling can remove thief skills at critical points of the game, however. The final level of Dragon's Eye is sheer frustration without an active thief - I've done it, but only with three clerics to shower healing and remove status effects on characters triggering traps that reset without a thief's trap removal ability. The DE lvl 5 locks are pretty well unbreakable, even with potions of giant strength, so you need the knock spell if you don't have a high open locks skill.

Evasion is gained at lvl 7. You could dual there if you like, as there are multiple items, potions, and spells that can temporarily raise your skills if necessary (Girdle of Gond from Orrick, Cat's Grace for increased Dex, Luck song/spell, various potions, rings and cloaks for better Hide skills). If you don't scout or backstab, put the majority of points in Find Traps and Open Locks - scouting and filching can be covered by other characters.

At higher levels and higher difficulty levels, a mage is worth its weight in gold. Web, Slow, Confusion, Cloudkill, and Acid Fog are only a few spells capable of changing the tide of an otherwise difficult fight.