Which character choice would be the best to finish the game with as few annoyances due to combat as possible?
Phantasm + Conveyance and Persuasion.
That's if you're not interested in combat at all.
As for spells, check on these:
Black Necromantic: Harm and Conjure Spirit. Note that Harm's effectiveness is overhyped - it's most effective against living beings, and less so against undead, machines, and all other stuff (and it does't work on inanimated objects). Conjure Spirit is a very fun substitute for Persuasion.
Conveyance: Disarm, Unlocking Cantrip. Teleport is not neccesary although you might want it to skip combat.
Earth: Strength of Earth, Stone Toss. First one allows to carry stuff and solve "pick boulder" quests, while Stone Toss is a damaging bread&butter spell like Harm, but works on everything and can be used to break doors.
Fire: Agility of Fire is probably one of the better starting buffs for any character. Fireflash allows to quickly dispatch almost any monster group (exept fire/magma monsters).
Force: Most powerful damaging school, but not very cost effective. Shield is a great protection spell, Lightning Bolt and Disintigrate do what they're supposed to do.
Nature: Charm Beast allows to skip 50% combat in game. Succour Beast summons most powerful summon in game if you have high magical aptitude.
Summoning: A very solid school with it's Guardian Ogre, see RK47's playthrough.
Temporal: The gamebreaking school. Hasten and Tempus Fugit turn any build (melee, backstabing, shooting, companions-heavy) into engine of mass destruction.