Make sure you look through the viewfinder of a P before buying. The P used "reflected" frame lines that may or may not have aged well. Some Ps' are fine, while the viewfinders of others show quite a bit of flare. The Leica M and Canon 7/7s use projected frame lines so don't have this problem.
Take a look at the Canon RF sub-forum on RFF. Lot of discussion about the P there.