With the batteries, you kind of have to pay attention to the physical size of the packs. Very few are alike in dimensions. I have Hyperion, Kokam, Maxamps, and TrueRC packs and none of them have the same widths. For a buggy, an 8Ah pack is a pretty tight squeeze. The Kokam 4800, flightpower 4900, and maxamps 5000 4S packs should fit nicely under the body. The buggy will handle better with these smaller packs too.

If you can, I'd suggest getting 4S packs, rather than trying to run two packs in series. I just think it's easier, although not as flexible as far as using the packs in other vehicles.

For 4S, whatever you do, get good packs. Flightpower, trakpower, Kokam, Maxamps (5Ah) are good packs and will deliver all the power that you need for a track setup.