Hello,
i would recommend a car internet sales platform and search for Boxsters in that country. By that you'll get a price range for the cars the sellers ask for. As we all know the real buying price is in reality below the asking price.
And than the car seems to have some problems. So i would substract the costs for that (done by a pro shop) from the asked price.
http://www.autoscout24.eu or
AutoScout24 could be a european platform where you could do your research.
Regards, Markus