One problem with converting RC cars into robots is most of them are much
too fast to control well from a microcontroller and typical sensors, like off
the shelf sonars. It's best to choose a car that drives fairly slowly, like a few feet
per sec max.
In regards a camera, you can get a wifi cam, or one that operates at 2.4GHZ
and has it's own transmitter built in, and a custom receiver. eg,
https://www.dynaspy.com/spy-cameras/small-wireless-color-camera-2-4ghz-audioThe latter requires a video capture adapter for your PC, or else a separate AV
monitor.
In regards RF link, you can get wifi modules that plug into reqular XBee boards,
so this allows you to experiment with either XBee or wifi. Eg, XVee,
http://www.mouser.com/rovingnetworksRNXV/