Well,
For the battery part, i thought they are all pretty much standard just as long as you keep the same amount of voltage to the board itself.
But if you want my personal opinion, I would aim for something such as an Amiga 3000, or even an Amiga 1200, just depends what exactly you want it to do.
Amiga 2000. I havent used one in real life yet, but I think its more to play the classic games.