$800 sounds about right for a PSP. Though the closer you get to $1,000, the more I'd recommend looking into a P7M8 (American mag release, larger trigger guard and heat shield, among other differences). A couple of months ago I picked up a P7M8 with 5 mags for $1200 (M8 mags are $60/ea last time I checked) so if you do the math, the gun with 1 mag came out to $960. I personally think the P7M8 is a better gun than the PSP, but I definitely understand the issue of price. They are certainly more expensive than many used pistols.
A "plumb" coloration of the bluing on the slide is normal and happened (IIRC) because of a glitch in the bluing process. If the slide is plumb, there's a very good chance the finish is original.
I haven't checked recently, but a few months ago when I was looking to purchase my P7M8, the PSPs marked at $1,000 had no bids. The only ones moving were the guns that started lower and hit equilibrium around $760-800.