Well, first of all, you have the brushless RTR kit correct? The speedo: XP SC450-BL is sensor-less so that will limit your options a little bit but for starters I'd look for a sensor-less motor (or a motor that can run either sensored or sensor-less) that is 13.5 turns. If you plan on just racing stock/spec class then you are fine with that motor until it burns out or you have cash burning a hole in your pocket

There are plenty of good companies out there but I know Team Orion has some good sensor-less motors. Also, Fantom Racing motors can work in either sensored or sensor-less mode and they just came out with the new Ion 3's a few months back. Those are pretty good motors as well.