I friend of mine has a 2003 F-250 with the 7.3L Diesel for sale that I am going to look at tomorrow. It has 75,000 miles on it, and I know it has had meticulous care (he is a retired heavy diesel mech...
The 7.3 was a good engine, but a little lacking in power, compared to later models. It should have many miles left in it. The transmission would be the possible weak link, especially if he upped the power on the stock engine.