If you want a high speed, yet fairly light and small weapon, May I suggest going for a cordless dremel? It's simple to wire one to a speed controller or other power source, and they also come with some nice blade choices, like zip cutters and small endmills.

As far as a driving system, I'd go with a vex tank tread kit, as it's not too expensive, yet it still has decent tourque and grip, as well as the fact that it's more maneuverable than a normal rc car (tank style turning).