There are a lot of things to consider when evaluating the condition of a used engine. When was it last overhauled and by whom? Not all overhauls are created equal. What parts were replaced and what was overhauled, especially cylinders and pistons? Ar ethee detailed records of measurements of parts kept? Where has the plane been stored since overhaul and how consistently has it been flown? Long periods of inactivity, especially in damp climates, almost guarantee cam and lifter corrosion. It is important to cut open and inspect the oil filter during an regular oil change to look for metal. Same for the sump screen. I also insist on a borescope examination of the cylinders to look for worn cylinder walls and failing valves. If there has been regular oil analysis, that is a big plus. From all this you can get a pretty good picture of the engine condition. Just performing a compression test and checking the oil pressure and temperature only tells you how the engine is running today, but it doesn't say much about how it is likely to be running 100 hours from now. For that you have to dig deeper.