For the diff, i sugest you to take the buggy ratio, that's what is more close r in ratio to the stock diff ration and you don't have to gear it hhhhhiiiiiiiiggger to reach an acceptable speed.

Batt: i have the flightpower EVO25, 5000mAh, 4s and i'am very happy with it.

Motor i've the Neu 1515, 1y with MM and work great.