Slipped Sleeve only affected a few months in 1999, not 1998. IMS rarely affects 1998 Boxsters. The only real worry for a 1998 is RMS, which you can detect because a leak will form before the engine failure and it is a few hundred dollars to have the RMS replaced. If I was you I would learn more about this, because you do not seem very well advised on these issues. There is no reason to spend $3500 replacing the IMS on a 1998 Boxster.
If you decide to buy it enjoy, it and don't worry about engine failure. The number of people shouting on the internet about IMS and RMS engine failure are not proportionate to the over 200,000 Boxsters on the planet, of which 99% do not have these problems.
The Boxster is a great car, I hope you enjoy them as much as I do.
