Jonomo,
There were a few issues with 97 and 98 Boxsters engines. I bought my 98 Boxster in 2003 and it has its second engine in it. By the time they are this age, if anything major is going to go wrong, it will have already happened.
Typical things that go wrong with Boxsters are:
Hood retracting cables snap.
Air mass sensors need replacing.
Servicing a Boxster is going to cost more than a Golf, but my garage tells me he doesn't like Boxsters because they rarely go wrong
He likes late 70's to early 90's 911 'cos he gets to do $9000 engine rebuilds on them all the time.
I get the impression that the 2.7L Boxsters from 2000 onwards have had most of the bugs ironed out of them, as well as better low-end torque, equipment and power. Probably worth spending up to one of these if you are concerned