1) Yes, a CA box will work on a TOP, they're interchangeable. Star mags aren't, they use a different setup. What's wrong with your mag? The guts are repairable, and none of the parts costs anywhere near a new one.
2) Best battery, well one that will run your gun I suppose. There's tons out there to choose from. My TOP M249 has an odd mix right now of ANGS, Guarder and stock internals, and I run it with an Intellect 8.4v 2400mah NiCad. Works pretty good. Higher voltage batteries work as well, but I don't see the need for them myself, as they put your rate of fire stupid high. It's hard to give supporting fire if you've chewed through all your ammo.
"Anyone with a name like Amanishourbariki should give a few letters to the poor Ng family." - Snarfangel, Fark.com