P3D has been 64bit since v4.0, previous versions up to v3.* were 34bit compatible.
You should still able to buy or stick with earlier versions of P3D if you wished to use older aircraft, although others should be able to clear this up if I'm wrong!
Hope this helps.
Flying: P3D v5 ~ Still Trying To Get My Head Around: X-Plane 11
Intel Core i7 - 6700K CPU @ 4.5GHz,
ASUS ROG Maximus VIII Hero Z170 Motherboard,
EVGA GeForce GTX 1080 Ti SC2 GAMING 11GB GPU,
G.Skill Ripjaw 16GB DDR4 3200MHz RAM,
Asus ROG Strix XG32VQ Monitor,
Bose Companion 2 Series III Speakers,
Thrustmaster Warthog H.O.T.A.S & CH Rudder Pedals.