Cars and classes are a very personal choice so I'd not put any opinion into the mix
However as far as transmitters go it depends on the frequency..if an RTR comes with an older 40 or 27mhz rig you could just buy a crystal based transmitter and swap crystals to your new handset. If however you have a 2.4g system then it's usually 'bound' to the cars receiver. In this case you would need a transmitter and receiver set to replace the whole set up in the car.
You can get 2.4g stick systems from £30...
J