Do you have any service records or a CarFax report for this vehicle? I wouldn't be concerned with that mileage on a diesel, but I'd still like to know how the truck was cared for. If you don't know what was done, there's definitely some fluids you'd want to change.
Have you driven other 7.3s? Or have friends with 7.3 experience? Just wondering how this truck drives/looks/feels compared to others you've checked out.
The price seems reasonable assuming the truck is in good running condition. Earlier this year, I paid 24,000 for a 2003 7.3 with 37,000 miles (pretty much the same truck you're checking). So 17,000 is a nice discount.
This site has a wealth of information. If you haven't already, be sure and spend some time searching or just browsing through the threads - they will give you a good idea what kinds of issues you might have.
Of course, as with any used vehicle, even if you do your best to be sure you find one in good condition, things can still go wrong. Last week, I had to replace the alternator on the truck I just bought. Only 45,000 miles. However, my 2 previous trucks were 7.3s and both had well over 100,000 miles before needed new alternators, so I'm hopeful this was a fluke.
Good luck with your decision and enjoy your new truck.