"Now to find a surveyor."
that should be two surveyors. You will find that most marine surveyors provide only a general observation about the condition of a marine diesel. You should hire an engine surveyor to determine the condition of the engine - especially including a compression test. For some engines such tests may be somewhat invasive and the seller may balk at having the engine pooked and prodded, but this is the only way to know what you are buying.
Remember that sellers know thier boats inside and out. What do you think happens most often when an owner recognizes, or is advised, that a marine diesel faces rebuilding? Faced with a $10K or more expense, many owners decide - sell the boat to an unsuspecting buyer.
Obviously most boats on the market have perfectly servicable engines. But some of them are on the market BECAUSE they face engine replacement.
Note also that a sea trial may be adequate only to recognize a seriously failing engine. For example, diifficulty starting a cold engine can indicate low compression can With a sea trial scheduled, the owner gets to the boat first, warms up the engine, and presto, it'll start on the first turn of the key...
Its funny how buyers spending 100s of thousands hesitate to spend $250 to cover the biggest risk of the boat purchase. I know, been there and done that, to the tune of $13,000.