+1 on the Slayer's models; they're awesome! Also, Billy's gun was an M-16 with shotgun underneath. Marauder Inc. has an M-4 that you could swap the stock on or cut off the shotgun and add to a basic M-16 (the route I'll probably take).

I'm a bit stuck on the legs, myself. They're all wearing basic BDUs, though knives and canteens and such would allow you to use different thighs. I keep waiting for an "a-ha!" moment, but it hasn't come yet.