I'm sure some people would argue, but I don't think the engine's performance is going to be effected VERY MUCH by the glow plug. Maybe a hundred RPMs up or down, but on a trainer, it's not a big issue. Actually, now that I think of it, you may want to get the cheaper plug for breaking in, and then some higher quality ones for your actual flying. As I said, quite often you will burn out a plug as you break-in the engine.

You can get computer radios that will allow you to choose any frequency you want, but they are pretty expensive. It's usually not a big problem to be on just one certain frequency (there are 50 available). If you know the place where you are going to fly, you can ask them what frequencies are used a lot, and then avoid those numbers (so you don't have to wait for somebody to finish with the one you need). Other than that, pick whichever you like! Like your age, or your favorite Nascar driver, or your shoe size, etc.