Price over convenience? I reckon you already got the kit lens, the 18-55mm to go with it. Then yeah, it's cheaper. I bought the D40 body with the 18-200mm and I love it. Not sure what's cheaper though, mine or your 2 lenses together.
I use Aerofly Professional Deluxe. I've tried Reflex XTR and it wasn't as good as. Reflex felt more like a game to me. But then again, these sims all about getting a hang of the controls no matter what direction the chopper is facing.
If you wanna fly it upside down, don't try it! Do it over and over and over in the sim, with as minimal movement as possible until you can do it with your eyes closed. 10 times.
Then I think it's safe to do it with your Rex.