Worst case is a new engine indeed. If the seller dumps the car, that's probably what it needs. Nobody sells a car at a steep discount for needing minor work. Now, I don't have a clue what a DIY replacement engine would take. I am sure you could get a used engine - I would call
Partsheaven New, Used, Rebuilt Porsche Parts - they can give you a price range, even if they don't have one available.
G