Thanks everyone for your responses! We've investigated a lot of these and concluded that most/all 150s listed under $20k will have either high engine hours, low hours flown per year (or sat for years), or some kind of damage history. At least now that we know what to look for, we'll be better prepared when good ones pop up in the low-to-mid $20s.
The valve lubrication was a strange one, thanks for the feedback. The record says the engine was running rough on takeoff, so they did a compression test, removed plugs, borsecoped the cylinders, cleaned valve gaskets, lubed valve guides, tested the plugs, and "washed down engine." Charged the guy $587. At least it runs more smoothly after that according to the seller.
Overall, it seems that recent flying for a couple of hundred hours does not alleviate the concerns caused by an engine sitting for a year or more. It may be running fine for now, but things could go bad at any point and the total life of the engine is probably reduced. Accurate?
