RE: Hanger 9 P-51 PTS
Personally I don't think these planes fly as well as they could in a training situation. I've flown easier to fly planes as trainers. However, since you are already flying a bit, it may be OK for your situation. Taking off and landing are what you'll need to learn, as well as slow flight, coordinated turns, stalls and the like. In a training situation it's best to have a docile plane that reacts a bit on the slow side so you can react to it easier while learning. This plane is a compromise between a trainer and a sport plane, so unfortunately you don't get the best of either. How does your instructor feel? If they are comfortable with it, it might be fine.
Personally, I would go with a plane specifically designed as a trainer and nothing more. Learn on it untill it falls apart. Then go and buy a sport plane. lots of things people dislike about trainers are actually good skills to master. Is it a terrible mistake to go with the P51? Not really. Just not my first choice.