Best battery?

[VintageMA]

What sort of speeds are you looking for, and what runtime are you going to try and get.

For racing you'd probably want to go with the 4S setup, and maybe get a 5S down the road for bashing. I don't know the gearing on the MBX5T, so I couldn't recommend the right pinion to get.

You could check out the TP 5000mAh eXtreme V2 cells here:
http://www.thunderpowerrc.com/html/extreme-V2.html

Or the Neu 25C packs:
http://www.neumotors.com/20061222/25C_Batteries.html

Both of these would be under $300.
Depending on the runtime you need you may not want to go with the biggest packs around - bigger packs mean more weight, which means more weight to move around and more Amp draw and hotter components. Hopefully some of the guys here who have some more experience racing can recomend a good pack.