[Perfectlap]

do not under any circumstances buy used shocks. I've had brand new ones fail on me after only a few months. Getting the Porsche back on the lift to replace a blown shock will cost you a needless expense.

I also don't recommend trying to figure out your own suspension. Call a Porsche tuner in your area (Maybe ask Brumos Porsche for a recomendation) and ask what they have that maybe you can check out and drive or if they've done recent work on a Boxster. But do this only after you've gotten in about 12 events.
After your first full season you should add up your experience and consider new tires and such. The tires are the key, put your dollars there and you could shave as much as two seconds a lap, which in Autocross is enourmous. Maybe a set of harness belts too. Being glued to the seat takes weight off the steering wheel which aids concentration.

The factory set up from Porsche on sticky tires is more than enough to get you fastest lap of the day without any mistakes on your part. When you reach the absolute limit (consistently within half a second of the quickest time in your class)
then consider modifying beyond tires.

P.S.
check out the EVOLUTION driving school. They are based out of central Florida.
Well worth the $350 or whatever they are charging.