I think all the good points are covered. The key one is to find a mechanic who really knows their stuff. They need to do the compression test, or a leak down test, and probably to use a borescope to take a peek inside the cylinders. Good
marine diesels will run 5-10,000 hours between rebuilds, so you're in that range where how the engine was treated really means everything. Looking around, or asking your mechanic, for signs of neglect or good
maintenance, mean more than the number of hours. Does the
fuel filter show sediment? Is the
fuel tank really clean, or has sediment built up in that? Does the engine start quickly and run well?
You might want to have the
fuel injectors pulled, cleaned and calibrated, (all routine at any diesel truck engine shop) and have a mechanic show you what is involved in properly bleeding the
fuel system, as well as changing the
water pump impeller, filters, belts, thermostat--they may all be recently changed and perfectly good, but you'll need to know how to do that for routine maintenance anyway.
I'd disagree on the value of an oil analysis. For a whole $25 it *can* often tell you if there's catastrophic failure to come, and it will tell you if the
current owner has recently changed the oil with a proper grade. Won't always tell you everything, but for $25 it can tell you if there's a red flag, like high levels of bearing metal in the oil.
Incidentally, there's no such thing as a 'Westerbeke" engine. They put out
contract bids for everything, with varying results, and then assemble the
parts from all of the lowest bidders and slather red
paint over the finished job. That doesn't mean a
Westerbeke is a bad engine--just that it really always has a secret identity.