He wants the sweet 7.3 Engine, most likely. I can understand that. However, if he were looking at swapping an '05 6.0 for a '03 6.0...I would think he's crazy. The other thing could be a much lower monthly payment as well.
What to look for? I guess the same as any used vehicle. I would probably have them change the ATF and filter, Coolant, and do a brake job for you as it's likely these haven't been done. Heck, make 'em do an oil change too (although that's easy to check...if the oil is golden, it's new oil).
Just know going in that you will likely need to buy new aftermarket shocks like KYB, Edelbrocks, Ranchos, or Bilsteins...and possibly even a rear swaybar from Hellwig. This will substantially better your ride.
Also, I'd swap out the 7.3 airbox for the retrofitted Ford AIS like what you have in your 6.0. It'll only cost you $200 and about 30 minutes to install yourself (not hard at all, just takes a few minutes).