You have to be careful. Like any other Porsche it is easy to buy one that you gotta spend $10,000 to make it a $5,000 car. $2500 sounds too cheap. Again, just like any other Porsche, does it run, Miles, Service history, condition of interior and paint, all the electric stuff work. There is a pinned thread on the 928 board on Rennlist.com that tells all the stuff to look for good and bad and the year specific common issues.
here is the link to the FAQ thread.