You can get started for a lot less than some of the guys qutoed above.

I personally like the Alpha trainer package. For just under $300, you get a very good rig that is one of the best flying trainers out there. After that, you can really keep the field support stuff cheap. A taped up wood dowl for a chicken stick, a tool box or even cardboard (A guy at my field uses an old beer case box, really thick cardboard) for a field box. Get a glow starter, and some fuel, and you're pretty much there.

If you want to save money on that, take a look at SPADs. It takes a little doing to build your first one, but after that, they are quick, easy, and cheap to build. http://www.spadtothebone.com

A 4 channel radio is just fine. 99% of the guys out there never use more than 4 channels in their planes, and never use the computer features. Sure, if you want to do fancy scale or 3D acrobatics, you're going to want a more advanced radio, but frankly, a basic radio is fine for a trainer, and it's fine for your second plane, too. There's a guy in my club flying a 1/4scale SE5a on a 4 channel Futaba, and it flys very well. A 6+ channel computer radio is nice to have, and I personally can't live with out my computer radios, but they really aren't necessary for the vast majority of guys out there.