What in your opinion are required dlc?
I just started playing myself, but in my opinion: "None".
Two of them DLC "Witch Hunters" and "Undead" are different races for additional campaigns.
So before getting into them i would finish the campaigns from the base game first.
The other DLC are additional recruitable characters which are only suited for specific races and not needed at all.
Some DLC charackters have already been added into the GOG version, because the GOG version lacks online features (which are not needed anyway if i interpret the steam forums right. Not many play this game multiplayer. There are no tournaments anymnore and such.)