I've had several 1999s that were all solid reliable cars. My daughter daily drives my '99 with a little over 140k miles. Rebuilt Tiptronic transmission, refreshed suspension, and IMS/RMS, but nothing else in it's history other than normal maintenance.
My first Boxster was well over 140k miles, abused by a couple previous owners, but still running great.
I bought
Retroman's high mileage '99 when it was having transmission (Tiptronic) problems at about 175k miles. I fixed a few things, put a couple thousand more miles on and re-sold it. Recently saw it for sale again well over 180k miles and for a couple thousand more than I sold it for.
ANY 20 year old car should be thoroughly checked and refreshed if you plan to run it hard, but I would not be more concerned about a 2.5l Boxster than any other car of it's age.
...and if you search
thstone's posts, you can read of how many hours he runs these engines at red-line, before he swaps in another
used 2.5l engine.