1. Get a code reader at the minimum, Durametric S/W even better. That way you can self diagnose CEL problems with the help of the online community.
2. Become aware of on-line guides to things like O2 sensors. There are
sites with tables of equivalent part numbers that are no-risk cheaper substitutes made by the guys who sell em to Porsche.
3. The Boxster is just a car. Don't be afraid to do the external things as a way of keeping maintenance costs down.