Sounds like what you want is a basic 2.4ghz radio. As far as low cost goes, I would look for a Spektrum 2.0 or a Losi 2.4ghz RTR radio on ebay. Thats about as basic as you can get for a 2.4ghz.
Something like this.
http://cgi.ebay.com/NEW-Team-Losi-2-...QQcmdZViewItem
or this
http://www.ultimatehobbies.com/istar...=SPM20200!1002
But there are plenty of FM radios that should work for you without glitching, but you will need to pay attention to others on the same frequency like with your stock radio. Many even run the stock AM radios without trouble. But, the AM stuff does tend to get more interference than FM. Any 2.4ghz should have no interference issues.
If you are happy with your stock radio function, try it. You might not have any problems at all.