You can build a T-Rex kit (or any kit for that matter) completely and install all the electronics later. That includes the motor and ESC. There is no component that you can't install at a later date. Your best bet would be to get a combo kit that comes with motor and ESC. It's cheaper than buying separates. You'll need 3 servos for the cyclic, 1 tail servo and a gyro. Then you'll need batteries, a radio and a receiver. If you need all of the above, expect to spend between $1000 to $1200 to get it in the air.