The mage is really a matter of taste, Freemage is more versatile as it can do a lot of shit, but it won't do anything well. That said if you GM light the crusader fairly early and use it decently as a support it'll work out fine, that's what I did at least. It is however annoying as shit to play since you need to open your spellbook constantly on the freemage to cast all your shit. You kinda want expert is almost everything so you get the cool buffs, then you basically become a buffbot+healer, shouldn't expect much dmg from that character. I made mine GM Prime though, get the Hour of Power buff and the Destiny buff to a high value/duration, as well as Arcane Ward to a good level, shit's pretty great to buff the rest of your group.
Druid is a much stronger healer and as such can basically solo heal, as long as you equip a paralysis ring(and potentially feeblemind too but you can always pot that). You can GM both earth and water and get insane damage reduction, so much that you can basically ignore any defenses other than health on the rest of the group. GM water liquid membrane is 25%dmg reduction for 9turns, stoneskin is 25armor for 9turns. Even with just membrane you can generally not have to heal during most of the fights so you can just DPS or do whatever.
Runepriest is the more defensive healer, equip the relic shield from the cartography quest which you can get as soon as you open act2 and he's insanely tanky so that it's very unlikely he'll die to anything in one turn other than spellcrits, which can be remedied by giving him all the resist barrels and potions. Dwarves also get naturally higher health pools due to vit scaling racial. You could go Light+Fire and get good damage too with strong heals, however it's not as hands off as the druid, you need to heal every other turn since you don't have any defensive buffs. You could go holy+earth but that's kinda pointless to go runepriest for that, might as well play druid then.
As for barb over merc, I'd probably do it I've already played a merc, it's not awful but it's not great. I'd do spear barb instead. Don't like hammer bonus and the 2H mace relic is in the last dungeon so it's kinda shitty.
