I believe it fits all the 986 models of Boxster, not the later years 987, so it should fit a 98. In order to install it, you will need rivets that hold the brackets you see to the rollbar, they should be a few bucks at any dealer, and I believe a pop rivet gun if you don't have one. It DOES have some scratches as it is used. No abuse though. If you were to buy it and didn't like the condition, you could send it back and I'd refund your money. I've included a link below so you can get an idea on the installation.
Take care,
Mike
http://www.renntech.org/forums/index.php?showtopic=5477