If you do any compression or leak down test. Loosen the plugs 5 or so turns, then retighten and run the engine for 5 minutes. I have seen false readings from carbon deposited on the spark plugs threads and as you remove the plugs, this carbon can get under the valves as you crank it. Giving you a low reading.
Another test to do , Is decelerate the car down a hill, then blip the throttle to check for smoke, indicating valve seals. And the opposite is accelerate in a really low gear to load the engine and check for smoke, indicating rings. Check the colour of the spark plugs. moisture in the
oil.
Good luck with your buy.
JD