The 99 with 57k looks like the newest with the lowest mileage. Check and see what similar cars have actually sold for. It also has a 2.7 vs a 2.5 liter engine, which you really need with that somewhat slower (and a tad heavier) tiptronic tranny. Upside? Very few RMS leaks on tip tranny'd Boxsters!
Good rule of thumb: The cheapest Porsche to buy is by far the most expensive to own. ... I didn't coin that phrase, but dang, I sure have lived it!
I do wish you'd keep looking for someone who has a 2000 S with really low miles on it and save up a couple of grand more. You would not be sorry with an S because of the 3.2 in it and the improvements they made every single year since the car came out made it better and more dependable.
I have 3x black boxster and love it... hard to keep clean and swirl-free, but when it's clean, it's the most beautiful color boxster out there in my opinion! Invest in a Porter Cable random orbital buffer if you buy a black one for detailing days.
Visit my web site (links below) and print off all the pages that talk about what I've replaced on my old boxster and what you should look for when buying one to give to your mechanic buddy when he inspects the car.
I would still take it to a dealership and pay them to do a $250 PPI on it. They just know what to look for on these cars where your buddy may not know all the nuances of an older Porsche.
Also, ask for the maintenance records on the car you are interested in to see what might have already been replaced and what you'll be stuck with after purchasing the car.