I like the old metal TL frame alot, plus it's a block frame so you can pull off the 45 handle and put on an 86*, eurogrip, etc if that's your fancy. I'm 99% sure your Piranha frame won't fit, the bodies are slightly different.
Side cocking is more Tippmann-esque, if that floats your boat. Depending on the body style, rear cocking may be a pain in the *** if you don't have beavertail. Top cocking is pretty popular although sometimes the cocking pins will fall out on older bolts, leaving you high and dry if you decock in a game.
If you're looking for just a beater blowback, pretty much anything will work. If you were thinking low pressure, a fatty body is your best bet as you get a little more volume to push around in the lower tube.