I agree with Nicholas fully.

One thing I would like to add: when you buy a used lens you don't know its history.
Leitz has been using the highest quality greases and lubricants over the years, expensive stuff, verry expensive and verry good.
If a lens needed a grease-change due to sand/grit and that was done by an independant repairman that didn't use the propper grease, you
will have a problem sooner or later.