I don't really know how much a rebuilt engine will cost, but I know that if you want to do more than bring it back to stock it will be more costly than a Chevy engine. Great engines though. Mine is from one of the big Olds' cars from 1974 and runs great, and it'* all original from what I can tell. The carb on it is also great if it'* still got the Quadrajet. And I'm sure it has a TH400 behind the engine, which last forever.
One thing you want to do before you buy this car is make sure it has all of the trim, and in good shape. Trim pieces are very expensive to replace if you are missing some. And a lot of times, especially with cars like that one where you don't see a lot of them, they can be hard to find. They probably don't even repro any of it.
2003 Black SLE-Stock
2000 Mercury Mountaineer
1968 Oldsmobile Cutlass Convertible-455 cubic inches of muscle