What mark do you have in mind?
Especially if you plan to play Nurgle Spawns (the best, at least for
40k) then my suggestion is to buy a box of Chaos Spawns and a box of Ogres from the Ogre Kingdoms of
WHF (
http://www.games-workshop.com/gws/catalog/productDetail.jsp?catId=cat670011a&prodId=prod1320015a)
With a very limited amount of creativity and ability with Green Stuff, you can create 8 perfectly plausible Spawns... they look especially well as Nurgle ones, since they will look a bit fat.
Double that and you will be able to field 15 Chaos Spawns (max available)
Another very good option is to wait for the new
IG Ogryn kit that will be released in the next two weeks and mix them with a box of chaos spawns.