I have had or should I say would have had a couple of Awesome sunset pics from my house over the last week. Untill I noticed acouple of trees that were alot smaller the last time I took photos The chainsaws also to noisey at night
I went to every second hand camera shop in London looking for a pristine copy of that lens. Because it is such a staple of the Canon pro kit, you simply cannot get one that hasn't been thrashed.
Having said that, the lens you are looking at looks otherwise tidy. If you can have a look at it, and it looks fairly clean go for it. Move the through the zoom range, defocus it to infinity, then auto focus on something close, take some shots and chimp for sharpness. If it feels smooth with a reasonable amount of play for its age then go for it.
Ken Rockwell is a bit of a poseur, but his review of said lens has an anecdote about the lens he is testing being dropped 9 feet from the back of an elephant onto cobblestones, only to keep working perfectly:
Duct tape over the cracked window for the pro look, but it may be worth looking for the part for repair. I have seen EOS service manuals with part numbers for every single piece of the pro lenses, so it may be an economical repair.