What do I need?

4 batts would give you a ton of voltage, but the weight would drag it down, as well as the cost of a system that could run that much........unless you're talking about wiring them parallel for more runtime, and then the voltage is more reasonable. However, the weight is still way too heavy. Just go with two IB 3800 packs, and if you need more runtime, check into Lithium Polymer packs which can be built to have 8000mAh if you have the cash.

As for the motor, generally a motor with 2000-2400 rpm/volt is best for a Maxx, but guys chime in if I'm wrong.