Both.

Start with the simulator. then start with a trainer. You'll soon find that the simulator is pretty good, but dosen't match 'real life' it'll help, but cannot replace 'real life'.

then advance to an 'intermediate' plane, then a 'sport' plane, or your P-51..

Most (if not all) warbirds are considered to be 3rd or 4th planes... (hi wing loading, stalling, nosing over, etc..)