A lease turn-in workout great for me; 2 years old, 18K miles on it for less than half of the sticker price. Of course, at the time Fords depreciated quite a bit--not sure of how it is now. But like most things, it's probably not best to generalize. It just depends on the situation.
I wouldn't buy a used car without service records. The lease return I bought was maintained at the dealer, and had 4 oil changes, which was the only service due. The previous owner even changed the factory fill early!
As far as "how much damage can be done", well, the answer is the same: it depends. My car came with a synblend, has a big sump and is easy on oil, and I bought it in SoCal, so if the guy had never changed the oil over the first 18K it wouldn't have been the end of the world. I wouldn't have bought it, but I doubt that significant life would have been taken off of the engine. 18K on a sludge-prone Toyota engine with no oil change? It could be close to the failure point. I think with most cars, you can do a good bit of permanent damage in 3 years or 30-40K miles.