Most of the guys i run with use between 8.5 and 13.5T motors, I use a 10.5T X-car hobbyking motor myself, its cheap and more than powerful enough. There is no hard and fast rules about what turn motor to use, just pick something in that range and you cant go wrong.
Tires: Tires are just about everything, you need to find tires that give you the right amount of traction for your setup and driving style, I prefer more traction so use hard rubber rather than hard plastics.