I have the 400 f2.8L and 600 f4 version II. Of the version I lenses I had the 500 f4, primarily due to weight. So the first thing I would do is to ask myself a very critical "Will I be able to carry this mastodont?".
I have not heard of any systematic faults with this lens. Optically it is still great. Beyond a physical inspection, I would stress the autofocus as much as possible and listen for strange sounds, hunting etc. I would also bring a chart of some sort, an ISO chart or something that could fill its role, to verify lens alignments and sharpness. I would also do an AFMA to check that it is within reasonable limits.
In my end of the world there are still very few of these available on the market and they get fairly good prices. Probably because their current owners are very happy with them.
Good luck!
