I agree with FHHuber on the radio.

Get a good 4-channel radio like a Futaba Skysport FM. Most likely, your second airplane(and possibly your 3rd) isn't going to need anything fancier than this. It comes with a 7 channel reciever so that if you move up to a .40 size airplane with a servo in each wing half for the ailerons, you can still just plug each wing servo into spots 1 and 7 in the reciever and it will be mixed already. I moved all my skysport stuff out of my Tiger Trainer into my Magic, and even though it doesn't have high rates it's plenty enough throw for me at this time. Be sure you're going to stick with the hobby before you spend $ on a radio system that you may never use or need.

For the engine, an OS .46 LA or Magnum .46 XLS will be just the ticket in a .40 size trainer. I'd go with one of these over a .40, the slight increase in power will be helpful in pulling you around the sky. I don't think a more expensive/more powerful engine is necessary, but it depends on what you're planning your second airplane to be. I can tell you that my .46 XLS has enough power with a 12x4 to give me unlimited vertical and a pretty strong pullout with the Magic, although I think it has a pretty light airframe.

If you definitely know you're going to be growing after the trainer, I'd put my extra $ into the engine rather than the radio system. A less expensive radio system can be made to work well with an intermediate airplane. If you don't have enough power with the engine, all you can do is try to lighten it, or end up buying a different engine.