Gotta agree with secondwind on this. My Yanmar 3GMF was 28 years old before I had a rebuild done. I don't know what the hours were on it. Didn't have an hour meter installed originally. I had mine rebuilt this past winter after I trashed a piston ring two years ago (overheating). Yes the engine key does hang on the raw water intake now!
Even with the bad ring, I ran the engine for a year, it just smoked, _a lot_, until it warmed up and than it ran fine. Yanmar is a good diesel. Like the saying goes 'If it ain't broke, don't fix it'.
Finally, as to costs, I paid $650 for parts. I still haven't gotten the labor bill from my mechanic, he's infamous for being bad about getting bills out. His original estimate was for 1.5 - 2k for the job including parts. I believe this is a very good price.