I support your choice of project as Mordheim is great!
For humans generally, consider Lead Adventure Bruegelburg. I have a Lead Adventure Bruegelburg order on the way I intend to make an Averlander gang with, and they could be used for many other things besides. There are also the Warlord, Foundry, Artizan, and other historical ranges to consider. Reaper could supply a large brute or two.
White Knight, moderator here, makes some great witch hunters.
Avatars of War make some nice witchhunters and many other figures suitable for Mordheim but I don't know if they can be had for less than what the GW ones go for.
Reaper makes some witch hunter types but they are a head taller at least than most GW Mordheim figures. If memory serves this vampire hunter from the Pathfinder might not be as tall, though, and is a nice figure:
Speaking of taller figures, and if you go in for T&A, there are the Raging Heroes Sisters/Daughters of the Orphanage, e.g.:
Noticed this other Witch Hunter the other day by Terrible Kids Stuff, who I'm not familiar with:
I see GW makes one currently as well, and for $16.50 it's a steal!
You are spoiled for choice for undead, and probably have those sorted. Obviously the GW and Mantic plastics. Reaper has some nice choices too, and I have some proxies in the gang I made up here:
http://leadadventureforum.com/index.php?topic=110149.0(And the gravedigger there could easily make a great, tall, witchhunter if you switch out the shovel).
For the possessed cultists, I figure you are planning to use the Frost Grave plastics, which are perfect for that. For the possessed himself, wrong-size base but this other Avatars of War figure is a nice possessed or chaos spawn:
I'm sure there are dozens of walking monstrosity type figures out there that would be better than the actual possessed figures, which are among the dregs of the GW Mordheim... I'll come back if I think of some. Should be easy to convert something too.