The P1 is a post-war P38 with an alloy frame and as such not particularly rare. The civilian models were designated P-38 even after the war. If it's a P1, it was a Bundeswehr issue gun, since that was its official designation. The German military had a huge number of P1s, they've just recently started to phase them out for the new issue P8.
P1s are good pistols, although they share the less-than-stellar DA trigger of the P38. They're also slightly tail-heavy, and many American shooters dislike the "safe down, fire up" type safety. I'd buy one as a shooter and for nostalgia reasons any day, but don't expect them to rise in value sharply any time soon.