Hum......My first airplane 20 years ago was a Sig Kadet Jr. Only lasted 3 flights, if you include landing in a tree a flight. I started building 2 channel gliders and flew them everyday......a while had pased and I built a Sig Kobra. It flew fantastic, really go where you point it, and landing was very predictable. I had a few crackups, but just cut it apart, and fixed it, no big deal. After using that plane as an avanced trainer, and forcing myself to learn, I got good, because flying that sort of plane requires your constant attention ( it doesn't self right at all, just keeps going where you pointed it last), and it built very well, nothing too hard, but if you pay attention to the details, comes out very nice, I even modified my next plane ,Sig Kougar to have a canopy that opened with a small servo. I learned to fly when the sims sucked and without a trainer cord, old school, so you either did or crashed and fixed.
With reguards to a P-51, if you like them enough, you probably won't mind having a few. I am building a Pica P-40 1/5th scale 86in wingspan,(among a bunch of other projects). I would like to say that I like the kit quality very much on the whole. Even though I had to make a few changes to it. It builds well, but the instructions suck, but then again its not for the first time builder or flyer. Top flight as well as Pica make a 1/5th scale Mustang that would be great practice on your way up to a 97in monster mustang. Remember when building, keep it light or it will sink really fast. To get some airtime when your ready for a Mustang, consider building either a .40 size or the bigger offering from World Models. The mustang gets pretty hot with a fast .45 or .46, but is pretty tame with a mild .40 on a bigger prop (11-5 instead of a 10-6) It is much less of a handfull than my Sig Kougar or Kobra for sure, with the only exception being the taildragger set up. I have a fellow flyer that has the 80in Mustang with a fox 2.4 gas in the nose, it flys really well, more like a sport plane than a scale job(tank) and the newer versions have flaps too, and goes for $499.99 from Quantum Models. If you price out a typical kit at 250 and add the retracts for another 400 and an aluminum spinner for 50.....your already at $700 bucks! For 5 bills you can have an ARF that I have seen fly very well that also includes retracts and takes a gas conversion well.
Although the retracts are not that good, once you can land without having to realign the struts, you've moved up another level. I think the best part is you can bump it up a little without crying like the first scratch on a new car. Good luck to your goals!