I would hold ff on the P-51 until the 3rd plane. Something like a SIG 4* or World Models Rambler, or GP Sporster would make an excellent 2nd plane. FWIW - World Models has 2 versions of the Rambler. The Rambler 30 is a fixed gear, while the Rambler 45 is retracts. Both use a 46. The Rambler 45 MAY be a better choice if your P-51 has retracts.