I recently built a B7D. Tight indoor clay track. I haven't raced in years, so I am starting in the 21.5 class. I went with a Reedy 600z-G2 ESC with a Reedy SP5 series motor, Flysky Noble NB4+ radio/receiver, Protek 575 servo, and a Trinity 130C-5600 battery. Those batteries are in the Horizon Hobby clearance section right now for really cheap! Seems like a perfectly decent setup for a low-power spec racer. I can upgrade the motor and ESC down the line if I want to race in another class.