Lenses don't really 'break' like that. If it looks ok and seems to work, it is ok.
Test that the lens focusses as expected and that it has no obvious signs of fungus inside or massive amounts of dust (a few specks makes absolutely no difference) and that there is no sign of major damage - again cosmetics make no difference to image quality - such as a zoom ring that doesn't turn properly.
The date code on the lens will give you an idea of the age of the lens, but again, it makes no difference so long as it is working. Focus motors etc can fail, but can be repaired and it is very rare.
Agreed. As long as the lens is in good shape, and the optics are solid, you'll be fine. I had an 80-200 f/2.8L for two years, and it was 14 years old when I got it, and it was flawless. My 85L is 5 years old (bought it a month ago) and, again, is flawless. With the Ls, if they're in good shape, they're likely to stay in good shape unless you abuse them.
Agree with Jman here. And in the case of the 'great whites' you can smack them around terribly and still get great results - I have a truly ugly (externally) 300/2.8 which produces the most amazing results every time.
The thing that breaks is on the cheapie consumer lenses is the expanding ring on ring of plastic as they rack out to tele. Such as 24-85 or less commonly the 28-135. Those two you mention are a internal tough guts type of lens. Some older discontinued lens can run into parts shortages and they will cut you loose, those two mentioned still current, you'd good for a decade plus or maybe longer maybe maybe....
With Canon's wide zooms in particular, you MUST check corner sharpness before purchase. In my experience they are prone to decentering, meaning you will see one side (or corner) softer than the others. When this happens, Canon is not always able to fix the lenses satisfactorily. I have had this phenomenon in varying degrees in a 16-35, 17-40, 24-105. (I had different problems with the best-forgotten 17-35.) The only Canon wide zoom I have personally owned and not seen this problem is the 16-35L II.
Sorry to sound so negative, but that's been my experience... YMMV.
