18awg would manage the gearmotors but will be too small for a brushless. You'd need the bigger stuff for a brushless setup as they pull a lot of amps. If you're gonna use a brushless make sure the battery can supply the burst current the motor will pull. I use cheap 3s lipos as all they do is spin a couple of drive motors, but if you're going to use a brushless you will need a more expensive battery that can give out the current it will pull.