The '99 is one of the MY's least likely to have an IMS failure (the failure everyone freaks about about these days). It is about as likely to happen to a '99, as any other car on the road, and only became an issue when they redesigned the IMS bearing for MY '01-05, and a few 987's.
The '99 is prone to RMS failure, which can be spotted as an oil leak before a failure occurs. The '99 is also prone to the slipped sleeve issue (cylinder sleeve breaks), however if it has made it to 98k miles that is extremely unlikely to occur.
Please don't ask me to explain the MY's that are affected by what failure. It is very well documented, although many owners like to replace the IMS on unaffected years anyway to feel 'safe.'
Enjoy the car, and realize that there are hundreds of thousands of Boxsters out there, mostly with very happy owners, and a few unhappy owners that had issues writing about it on the internet.
If you still have buyers remorse, go drive down the twistiest road you can find and report back!
