Assuming a good model, starts well, no smoke good compression.
Is there anything that would put an engine on the watch-out list for you?
Age, external corrosion, hours, rebuild?
Anything, what would worry you?
Here is how I would look at a diesel engine normally:
Listening to the engine will tell you a lot if you know what to listen for. While you are listening, you should watch the exhaust color and have them go up to full rpm both in gear and out. When it is in gear, you will also be able to tell if the gearing/prop is correct. Blue exhaust means that it is burning oil which means it isn't sealing well. If the engine is cold, this might be normal but if the engine is warm, it is a sign of worn rings. Black exhaust means that it isn't getting enough air for the amount of fuel. This could be because the engine is lugging or the air filter is plugged. Grey smoke means that the combustion in the chamber is not optimal and is a sign of fuel delivery issues normally. If diesel or oil comes out, you should look into it.
If they will let you, I recommend taking an oil sample and sending it out to a place like blackstone labs. If they will tell you how many hours are on it since the last oil change, you can get a pretty good idea of what condition the internals are in.
A visual inspection is definitely worthwhile. Look for oil leaks and coolant leaks. The hardest leaks to deal with are usually a rear main, sometimes the oil pan and the head.
How it starts will tell you a lot. Both cold and hot starting are informative. Lots of cranking on a cold start is a sign of poor compression or an air leak in the fuel system usually. On some engines with glowplugs or grid heaters, it can be a sign that these are not working.