Looks sorta ruff, but decent. I would talk him down some.
Check the starter. Make sure it hasnt broke off the ear on the block. I have seen several do this because people dont install the support bracket back on.
Make sure you start it cold. If its hard to start when cold, could indicate bad glow plugs, relays, worn injection pump, low compression. If the engine is warm when you get there, suspect the owner trying to hide a starting problem.
6.2's are great engines. In fact, I would rather have one than a 350. Main problem is hard starting. If everythings in good shape, motor should be ok. Check maintanance too. If it wasnt taken care of, be cautious. If its been properly maintained, I wouldnt be afraid of it having a lot of miles. I have had many 6.2's in excess of 300,000 miles with no major work done.
Use these things to help you talk him down a little.