Well I like both ideas. Witch-hunters and mercenaries. We could combine those two ideas. Maybe some prominent and pious (zealous) Sigmarite and witch-hunter decided to have his own little crusade against enemies within Empire and for that reason he assembled a group of same minded individuals + paid some freelancers to have some extra muscles. 
As for races I think that only humans and dwarfs would be really possible maybe with halflings from Moot.I am not that sure about HElfs, person playng that char would have to have a good reason to be with group.