Boats with motor problems???
I''m a little taken a-back with considering a naturally aspirated diesel being ''old'' or near needing a rebuild with only a couple hundred hours on it. The biggest thing that will kill a 80''s diesel is the owner(s). With proper maintenance & clean fuel you should get service into the thousands of hours rather than hundreds. Naturally, if the engine has received poor maintenance then it may die very early. Take a look at the maintenance of the entire boat...it may well reflect the care the engine''s been given.
Make sure the engine oil has some hours on it & then get an oil analysis..it''ll run you 25-30 bucks. I''d look at the analysis as a first pass check on the engines condition as it can tell many things about the engine''s health. You might also pick up a copy of Nigel Calder''s book "Marine Diesels".