Just scroll up a couple of posts, Crius posted the links for the top and machine gun.
Any 1/6 scale (12") figure will work without the top on the Jeep. With the top a 11" figure will be better or you need to make some modifications. Somebody on YouTube recently posted a video where he replaced the seat cushions with thin cloth pads. I am still looking for a nice figure myself, some EBay sellers claim their GI Joe figures are only 11". I'll probably try that.