Welcome to the forum... as you see you can get a lot of free advice here... worth every bit you pay us for it! I learn a lot from communities like this one. Just to add more thoughts that have not come up:
One common question that I've asked is, "Should I get an Engine Surveyor in addition to the hull/systems Marine Surveyor?" The answer I usually get is, "It depends." In your case, the answer would be, "Yes."
Full engine oil analysis for engine, genset, and transmission is commonly done by any type of marine surveyor. But anyone can tell you that point-in-time report is not nearly as useful as having a long record of these analyses done say every year or oil change. SO, simply tell the brokers (or your own buyer's broker) that the maintenance records must be thorough, and include periodic oil analyses, or else you are not even interested in looking at the boat. That criteria weeds out a large number of available boats, (even many good ones), but does not cost you anything, compare that to a repower!
Just as our physicians consider thorough blood work as an essential assessment of our health, the oil analysis history tells a story too.
Chance favors the prepared mind! (Ben Franklin)