A good used lens is a good lens and thats that.
There are plenty of lenses out there that are over 50 years old and still going strong. In manual focus lenses there are very few moving parts, and they are not moving fast so the rate of wear is minimal.
Modern lenses have benefitted from new glass recipes, new coating technology and ever improving actual glass element designs. Manufacturing has become cheaper with very accurate mechanised grinding and polishing. This level of accuracy and very, very good glass was available in times gone by, but the cost was prohibitive in comparison. Mechanisation has made slight improvements in optical rendition and transmittence, but it is the cost factor that has really been improved. Just look how many people can afford the top end glass these days - because they are more affordable than ever before.
Autofocus lenses are driven by powerful electic motors. They have more moving parts, but again, in the grand scheme of things a used lens, especially from a very keen amateur user, will have only done the equivalent of something like 1 year of professional use. That is nothing for a lens that has probably got something like 15 or 20 years of average professional use built in. Don't be afraid to buy a good used lens. Look for matching papers, box and other evidence of legal ownership, thats all. The outside look of a lens will tell you if it has been well looked after. The faces of the optics will also soon show up any scratches simply by letting light play on them - if there are no scratches it is fine. Smears might simply be grease - try wiping VERY GENTLY with a lens cloth, if it moves it is just something on the surface that can be cleaned off, so don't worry about it.
Repairs are readily available and not necessarily expensive. Optics can be repolished and coated - or replaced. This will be the most expensive kind if repair. Bayonets can be replaced for about £30 (on mine anyway) very easily - 3 watchmakers machine screws. Job's a good'un.
You can get much better lenses by buying used than if you buy new - probably get professional quality makers lens for the price of a new other makers lens.