Just some advice from experience made in 17.5 turn off-road racing: chose a single spec motor. There are huge differences between various manufacturers. The Graupner Sport motors (should be available through Schumacher as your GM Racing distributor) are just insane: 395 watts from a 13.5 turn motor compared to e.g. an LRP Vector X12 at 241 watts...
We chose the Speed Passion Competition 3.0 17.5R for our Short Course Spec class, which is comparable to the Slash's Titan 12 silver can. The LRP X12 17.5, on the other hand, had much more torque. Take your time to find a motor that is really comparable to your current motor, and the racers will love it!
