Yeah, I'd suggest just buying a BLI P70. They are exactly prototypical, with correct underbody parts (like A/C), the right font, decent trucks, and with MTL compatible couplers. They're easily equal in price to buying old cars, decals, paint, couplers, steps, roof vents, trucks and the time and effort needed to build an almost-sorta version of a P70. The Lima cars also have a deep groove down the side that can't be filled in seamlessly.
BTW, looking at the VGN plans the BMP-1 looks exactly like the Pennsy car of the same type.