as for the donor vehicle, consider grabbing the plates off your car and using them for the drive home (I've done this before). if by chance the plates get randomly run, it'll say 93 Sentra and chances are the cop will just assume the donor is that 93 sentra

If you do end up getting pulled over, having a bill of sale or something from the seller could help your case. Explain he didn't have the title to give you yet, but you just picked up the car and needed to drive it home.
if the cop is cool, you're in good shape. If he wants to be a hardass, then you're in not so good shape
how long of a drive is it from the seller's place back home?