I've got an Interarms PPK/S that I picked up about a year ago. It was in excellent condition and went for $375. $500 sounds a little high to me for that firearm. It's been 100% reliable the 3 or 4 times I've had it to the range. Good tight groups at 10 yards, too. It will bite the web of your hand where you grip it if you grip it too high. After my first range trip with it, I took the slide off and rounded some sharp edges on the bottom of the slide where it travels over your hand.
I bought one of the new S&W PPK/S when they first came out. They extended the beavertail on theirs which helps with the slide bite. I sold it to a co-worker that wanted it more than I did at the time for $375. I can understand your jonesing as I eventually had to have another.
I did order a Wolff spring kit and lightened the heavy trigger up a bit.
In regards to the PPK vs. the PPK/S, all dimensions are the same except the PPK/S grip is 1/2 inch longer to accommodate one extra round. That extra 1/2 inch translates to an extra 1.6 ounces in weight.
Most folks I know either love them or hate them. I'm pretty fond of mine.