One of the more recent limitting factors for budding new Spitfire\P51 pilots is................Insurance. If they dont like the look of you you wont get in. As stated above relavant experiance, a clean record and having a warbird operator take you under there wing is essential.
Large wads of money are essential as you need to get the experiance yourself eg All aspects of Chippy & Harvard flying, the Crazy Horse FULL graduation training plan, other taildraggers, aerobatic types. Furthermore be willing to muck in with the rest of the team who proberbly arent all pilots but have proved themselves with their dedication to these machines - NOBODY likes the prima donna "......but Im a pilot, I dont clean squashed flies off the prop". If its ok for Charlie Brown to clean up its ok for you!