At minimum a motor, an ESC (speed control), a servo for steering, and a radio and receiver. Nitro cars swap the motor and ESC for an engine and use a throttle servo.

Match the ESC’s current rating and cell count to the motor and battery. A brushless motor needs a brushless ESC of the right size.

Some are rated water-resistant, many are not. Check each item, and protect non-rated electronics from water and heavy dust.

Yes, upgrading the motor, ESC, servo or radio is common. Just make sure new parts are compatible with each other and your car.