For Part 91 operation, you're fine going over TBO. Like the others said, the PT-6 is a solid engine and will not likely come apart. What you will see is deterioration in performance. The danger here is your climb performance on hot days - especially at high elevations. When dealing with a potential purchase, you should definitely take into consideration the fact that it's over TBO when making an offer, but don't necessarily let that kill your deal.

It would be a good idea to plan on having a Hot Section done in the near future if you do go through with the deal. But at minimum, have a reputable shop do a borescope inspection on it and they can give you an idea of what kind of engine you're getting. (if you're near Oklahoma, I'd recommend Covington Aircraft in Okmulgee, OK)

Another thing to look into is the MORE program for PT-6's. I used to maintain a King Air with -20's that were nearly 8000 SOH. But they were on this program that basically tracks the life of the engine through more frequent inspections and regular trends. The program is mostly for 135 operators, though, I believe.