The general rule of thumb with Boxster's is to buy the lowest mileage and newest car you can afford.
If you want to spend between $18k and 21K I would look for a '00 or '01 and you can probably find one with less than 40k miles if you hunt. It depends on what your looking for. If it is going to be a weekend car then a newer higher mile car might be okay, but if it is going to be a DD I would go with an older lower mile car. A car without a lot of miles is okay, I got my '99 with 17k miles, that was less than 2k miles a year the previous owner did. I had a vaccum leak from some crystallized fuel, and after that I have had no problems related to the low miles on the car.
With 68k miles your probably going to have to spend some cash in a few thousand miles, on the things like rotors, etc.
If you love the car and the PPI goes alright go for it, but I wouldn't rule out continuing to search for a lower mileage car. I think 18k would be fair but you might be able to go lower considering the market for cars these days.
