Yes, you will need a ubec. As for a starting point, I'd start with the stock gearing and work from there. If the motor is hot, but the esc is cool, that would generally mean that the motor is under geared, so you could go up some with the pinion. If the controller is hot, but the motor is relatively cool, then you are probably overgeared. Go with a smaller pinion. If the motor and esc temps are pretty close (15 degrees or so) you are probably in the zone and where you want to be.