Great choice in planes. You will love the P51 pts. Its alot of fun to fly. I too bought the flap servo and installed it when I built it. I figured that way I could set the flaps any way I needed without having to remove the wing to move the wire for the fixed position. I soon found, I should have just used the wire. I have never used the flaps. The plane lands slow and really nice without it. Especially with a head wind. Live and learn. Although people say its not a trainer, I soloed after 9 flights. It was quite easy to learn to fly it. I, like you, came over to planes from racing cars. I spent alot of time on the sim and all of my instructors said they could tell. They can always spot someone who has sim experience. I now have 3 birds in the hangar and im looking to purchase my 4th. I figure the addiction could be a heck of alot worse then planes. Now I have to find a place to store the darn things.
To save a few bucks, the AR6200 receiver would have worked well with the mustang. I put one in my twist 40 and it works just fine. Not that you did anything wrong. If you buy more and dont need all seven channels, its an option in the future for you.
Now get them built and go fly.