Sorry for the late reply. I didn't see this response until now.
A digital unit doesn't requite crystals and you don't have to worry about interference from other radio units, like the FM/AM frequencies such as the one you have. The digital unit will also have a much shorter antenna, or a built in one. That feature is really nice.
I would recommend at least a 3 channel. You can get more channels if you like. Those extra channels would be come in handy if you add lights or other electrical add-ons.
A good company that has inexpensive units is Spektrum. Check them out and make sure the one you look at comes with a receiver. You will see a lot of remotes that are pretty inexpensive, but they don't include a receiver.