As for the bonus, the Dark Priest is definitely the best. You just need to manage to level him up without losing his class during combat. When I played the first campaign for Gauldoth, I chose this class and took 2 life magic skills at the castle's university on the second map. However, that ability turned out to be almost useless because half of the opponents were undead... it's not very fun to play as undead against undead in single-player.
But in multiplayer, more than half of the classes are either not relevant (unattainable) or leveling them up is not worthwhile, which is a shame...