The 7.3l will run forever if you change the oil every 10k or so and clean/change the air filter at the same time, same with fuel filters. I bought my 97 F350 single wheel, regular cab XLT new in Nov 96 with 3 miles on it. It now has 353k on the clock. It has gone thru 3 transmissions(auto) a vacuum pump, water pump, lift pump and a few sets of glow plugs and front wheel bearings. I have been running 285/75-16 tires on 10"wide wheels and I get 40k+ miles while only rotating them front to back once in their lifetime. The body has has zero issues except for the screws rattling out of the door panels every 10k or so. Drive train is without issues, axles are damn near unbreakable on the 350's(dana 60 front, sterling 10.25 rear, 10.50" on the SD) Of course the 250's had the dana 50 or 55(?) TTB fronts that were ok but not great.
Everything on my truck that has been replaced has been normal service items with the acception of the trans. Keep the tanks off empty, change/clean filters and oil, drive it and be merry!
And on milage, alot of people do over-estimate their milage to sounds more badder-asser but in reality the old and new 7.3's get around 15 on a normal basis. That would be average. On back roads under 55mph, I get upwards of 21-22. Down the freeway at 75+ I get 17 empty and 14 loaded, 12 if there are alot of hills to pull. That 12mph figure is with my old race trailer and car(28' enclosed 10,500lbs loaded) It depends on how you drive it, Stay off the boost as much as possible and milage goes way up. That's the nature of the beast.
Hope your truck treats you well!!!