I have bought many used and new lenses over the years. Both manual and AF lenses. Many of those lenses were before the internet really got powerful like it has in the last years. So good old fashioned books (what turn pages, what's that) helped. Moose Peterson was a forerunner in the nikon world in really writing the truth of old glass. Recently my friend Dean who went with us on last years TLT in Moab sent me an article on my 500f4P telephoto lense. The sight he used, and he uses many, is www.nikonglass.blogspot.com
. Wonderful right up on my lense. I still believe in f2.8 lenses if you can afford. Usually the best optics and coatings, smoothest mechanisms, and most money. But now you don't limit yourself from an exposure standpoint. Yes I know, now with you can reset your ISO at any time instead of for the whole roll of 36. But with f2.8 usually one less gyration to go thru and you can keep your ISO lower.
Also I am not very kind to my equipment at all. Even though it is mostly highend. So once a year or year and a half or so I would take to to Nikon Factory Service in Torrance,CA to have it serviced. Now this is not free but one thing you have to remember. They cannot or will not ship it back to you until those components are back to factory spec. This was very important to me as a photographer and an owner of expensive and abused equipment. They send you an evaluation with costs for you to approve or dissapprove and you go from there. Local guys being what they are I believe in the mother ship.