Before you all go and flame on me, I have been instructing for 15 years, and worked with three students with the P-51 this past summer. One has done OK, one is still working on landings (banged it up a bit), and another sold his and bought a high-wing trainer.
And I was just about to light a match[:@]... Not really
Good to see both viewpoints. Keep in mind that everyones favorite trainer is the first one they were successful on. Me... it was the Nexstar. Many people say it's to much money on training gimmics. I agree with them now as I removed the gimmics quickly... AFS never got off the ground.
I would recomend you buy the least expensive trainer you can get and if after you learn to fly you still want a P-51 then go get a Sig 4-Star. And after you have mastered that if you still want a P-51 then get one.