I have a friend that's a 928 entheusiast (he has two, an 84 and a 92). He spends ~ 3 thousand a year in maintenance (each). He's on all the lists and clubs, he's very knowledgeable about the car ... he does some of the minor work himself (and does an excellent job).
They are very comfy, very stylish .... but very expensive to maintain. There is no such thing as a cheap Porsche repair (or maintenance), and 928s are fairly maintenance dependent. If the seller hasn't been maintaining it "right," you'd be buying a massive cash drain.
I like 'em, but they're not cheap to fix and keep.
FWIW
Scott