Personally, I think if you have already made it to over 100k on your car, I don't think you have much to worry about. I am part of an informal Northeast Tri-State Boxster club that gets together for wrenching and drives (
www.tsbl.org). We have over 30 members, and I cannot think of one person in the group who has had their engine fail due to an IMS or D chunk failure. One person has had an engine replacement done by the factory on warranty due to an RMS leak, but that's it. I have just hit 60k miles on my 01 Boxster, and other than regular oil changes (always used M1 0w-40, btw) & brake jobs, I've only had to replace the coolant tank, a worn motor mount, and a water pump. Pretty darn reliable if you ask me.
Now I'm not trying to say there isn't a weak point in the M96 engines, as I think there obviously is if Porsche engineered out the IMS, but for many owners, I think they will never experience a problem. Just continue driving it and enjoy it I say, and worry about a problem if/when it ever materializes.