I have repaired several Boxsters myself and this one I would probably just use as a parts car.
My choices in order:
#1 Sell it at a loss. More money into it is just more money lost. For all the money that you would put into making this one right, it probably would not significantly raise the value above what you paid to buy it.
#2 Fix the absolute minimum needed and enjoy it while it lasts. Is it safe and legal to drive now? Can a small investment and a few hours of you time keep it roadworthy for another year or two?
#3 Arrange to have it stolen. Many years ago, I had a friend who owed a lot of money for his car and could not keep up payments. First he tried parking it near the ocean when a storm was coming. The storm arrived at low tide and the car was fine. A week later he parked the car in a bad part of the city and
forgot to lock it. Two days later they found it stripped out and burned. His insurance paid him the 'book' value of the car.
***Just saw your reply that it is not rust on the body, only exhaust etc... I agree with Geof3 that most of that is just maintenance that needs to be caught up.
Check out this thread:
Bad news from the mechanic - any insight greatly appreciated!
Original estimate for repairs was $15,000! speedyspaghetti is now fixing the car mostly himself and seems like it's working out well.