Presumably your purchase is subject to a test sail?
I'd suggest doing the following on the test sail, then deciding whether you want an engine survey :
Check the state of the oil on the dipstick. Not too dark is good. Rub a bit of the oil between your fingers. Does it leave them sooty black? No is good. Yes means the oil is old and the surfactants are saturated.
Take the cap off the heat exchanger, dip your finger in and have a look at the coolant. Should be a nice blue or green and not rusty.
Start the engine from cold (check it's not been warmed up.) Should start with very little cranking after 30s warming of the glow plugs. The smaller the whiff of blue smoke from the exhaust on initial start, the better.
Is is now running smoothly and responsively?
Try to do at least a 10 min drive on the engine, enough to get the coolant and oil up to temp. Then check for blue or black smoke at full throttle.