I can't comment on the Magnum, but I'll echo other thoughts, I'd buy a used, but decent shape .40 high wing trainer and slap the .40 on it. This way you get a forgiving airframe and can fly a nitro and get a feel for it. You said you can fly foamies, but haven't said what kind/type/style of foamies.

Then while you're flying the trainer, save up your pennies and get a good engine for your Mustang. Good luck!