I know this is what someone always answers to radio control queries, but the cheapest, most legal, and best functioning way is usually to just use a premade radio control kit. You can still customize the features of your project all you want, but I have found using premade transmitters, receivers and crystals, which already have all the legal stuff worked out, to be far better for my purposes at least. I've been working on methods to use these recently, infact in my blog I now have PIC code that could easily be adapted to any 18F chip that has a CCP module to capture the PWM servo control signal from a common radio control receiver. If you are just controlling positional motors you could even just use normal servos, and if you want to control the speed of a motor, you could use my code and set up your own PWM controller for the motor around it, or you could buy a premade motor controller that plugs into your receiver, they make lots of them. If you really insist on making the radio equipment yourself it can be done, but getting it all approved and legal could be a long and expensive process, to say nothing of the difficulty of making it work.