actually the reciever and 'ESC' are included together. you plug your modified servos directly into the reciever. or if you use simple DC motors, you could also plug the motor controller into the reciever (depending on the motor controller input capabilities).
and you need a battery (up to 6V, unless you use a Y-harness for higher voltages) for the reciever, and i think 7.2V for the transmitter (usually comes with the transmitter).