The plugs are easy to replace. The O-rings look to be easy too and a cheap replacement. If you go to the Boxster maintenance section on my website there are instructions and required tools for plug replacement. The most important thing is to get the
correct spark plugs with the proper heat rating of 7. I have the part number listed on the site, just don't settle for the plugs sold at the local auto store. The site linked above have '97-'99 for the plug, but they are actually for '97-'04.
The O-ring instructions say you just need some expanding pliers (the ones that open when you squeeze the handle rather than close and grab) to put in the plug tube and pull it out.
Whatever you decide to do, you should get the Bentley manual so you have some kind of reference while you work on things.