Very good choice for your next addition to your classic gun collection, Sailor! Ghostie has quite an impressive P7 collection. Send him a pm and ask him to repost the pics.
Price is all over the map and dependent on the variant, whether commercial or ex-police issue, number of mags, condition etc. However, don't expect to pay less than $2k for one and even that is a conservative number.
The PSP is the original P7. It has a push heel mag release ( vs. the more conventional pull type) and lacks the plastic heat shield at the trigger guard. The gun would become very uncomfortable to shoot after a few mags as it heats up quickly.
The M8 is the 8 rd. updated version ( I believe in response to the New Jersey PD requirements, but I could be wrong) It has the heat shield and an ambi lever type mag release in the conventional position. The M13 is the double stack version which holds 13 rds.
There is also an M10 which is a .40S&W variant. It is a chunky gun and definitely rarer but for some reason, doesn't command the price I think it should.
The rarest versions are the .22LR, .32, .380 convertible called the K3 ( 3 calibers) and a Target version. These are the Holy Grail of P7 collectors. And will command prices in the $3k+ range.
The gun is heavy and the loud clack it makes while de-cocking makes it a poor choice as a stealth CCW firearm IMO. It is however a fantastic piece of German engineering. You will not find a better fitted factory gun of that era.
I wish I had kept my M8 but I traded it for a top end AR straight across and, even though I miss the P7 due to ts rarity and novelty operating system, I really love my AR!! So a good trade!
Good luck and hope you find one for a price you can afford!