Just my 2¢.

I'd spend money on a good radio system (Spektrum DX6i comes to mind) and a good engine (Thunder Tiger .46). These are things that can and will last you a long time. Then try to pick up a used trainer at a swap meet or LHS or craigslist, or wherever. You could get a good used trainer airframe for super cheap (if not free!). Then you can put extra money into the support equipment (field box, fuel, glow plugs, fuel pump, glow plug igniter, etc etc).

Then once you're ready to move on to a new plane, you can take your radio and engine and put them in the new plane. Problem with a lot of the RTF is that the equipment is good, but not great. Meaning that sooner or later you'll want to upgrade. So might as well upgrade from the get-go.