"Is owning a Porsche (2001 Boxster S) something that is too expensive to be my primary car?"
Are you considering buying one from a dealer with warranty or a private party? If you buy one from a private party, make sure you get a Pre Purchase Inspection (PPI) by a Porsche mechanic. As you saw from postings on different forums, catastrophic engine failure is a possibility. What percentage? It seems no one really knows or is telling. My advice is try not to spend all your extra money at once on aftermarket upgrades and try to bank some money in case of a really bad day.
You are your own best judge on what your mechanical abilities are and how long you can tolerate your primary car being out of commission. I suggest that you read through the many pages of postings here, pelicanparts.com, ppbb.com and at
www.renntech.org to get an idea of the types of diy work performed and common failures. Try getting a copy of a workshop manual through somewhere like
www.workshopmanuals.co.uk or a subscription to
www.alldatadiy.com. A repair manual for the Boxster is just being released:
http://www.foreignintrigue.com/specials_html/986/index.htm
Caveat - nobody knows how good the repair manual is.
No doubt Porsche parts are expensive but you can use the internet to your advantage to search for good prices. If you get a Boxster, consider joining the PCA in your region to help establish contacts to help find where the good independent mechanics are in your area, for those jobs that are beyond your scope. Of course, the PCA can help teach you how to drive your Boxster better.
If you are lucky to be in some area of the country such as San Jose, CA, some forums organize DIY days to get together and team on upgrades and repairs.
Make sure you talk to your insurance agent prior to purchasing a car so that you don't get any unnecessary surprises in that area.
You're doing the right thing; go into Boxster ownership with open eyes and your homework done. It'll make driving your Boxster that much more fun.