Welcome to Librarium Online!
What are you guys using? I can't think of a model (plastic preferably, I can't stand metal models) in GW's range that would perform as a decent base for a grotesque conversion. I've been suggested rat ogres but they're not only expensive but hairy, and that is never a good combination.
Ogres/trolls are too fat, chaos spawn are too.. err.. chaosy and most of the other medium-monster models aren't humanoid (eldaroid?) enough.
The best conversions I've seen used the Island of Blood rat ogres, like these guys: Island of Blood Skaven Rat Ogres and Packmaster - eBay (item 400180873549 end time Jan-12-11 21:22:37 PST)
The heads were replaced with Ghoul heads (though I'd probably go with some of the helmted Wyche heads), and the fur either filed down or painted like eldritch flame. The regular rat ogres aren't nearly as impressive, and are more problematically hairy, but I don't think you could find a better base than the IoB ones.
The heads are dead easy in a way - file them flat and green stuff a mask close to the codex picture on.
If you've ever played the video game Dead Space (clicky for picture) the main character's helmet should provide some inspiration.
Heads aren't a problem, I've got the helmeted heads from the jetbike & wych kits kept aside just for this purpose.
However the torso & legs are giving me a real headache (not so much arms). Rat ogres from island of blood certainly do look easier to remove the hair from, it's a shame they're 2 part models though, it'll make having any variety in the unit that much more awkward.
It's also a shame that the new Minotaur models are hideous (and hairy). Keep the suggestions coming!