Leveling a duo will also give you a third high level character eventually. Follow a quest guide, but skip all collection quests. If you get too far behind (which I don't think will be a problem), just run to an area not in the guide and do a few levels.

Mage/paladin is a pretty fun combo I think. The paladin can tank a lot of mobs while dealing AoE damage. Run around to gather mobs, when you have enough, run back to the mage and Arcane Explode!

Also, the guy with the plate has the resurrection ability.

edit: You're wondering how much xp comes from questing and how much from mobs. I was fully rested and it took, I think, five or six levels to chew that up while questing. That was with three characters, so a little more group bonus. Maybe 50/50 from quests/mobs? Someone else have a better figure?