I would stay in the 7.3L era... while not perfect, it is a very durable engine that has proven to be able to work hard for hundreds of thousands of miles... We have one in the family with 350K+ on the clock.... has been driven by everybody, It used to be maintained very well, until it got semi-retired and passed 325K... now, its neglected/abused but will still start and run... She needs a pump and some injectors, but it will still get to town and back...
We have not been so lucky with ANYTHING that Ford has built since.... may not be everybodys experience, just ours.
As a side note, parts are readily available in the aftermarket for the 7.3L... my S.I.L. also has one and recently lost the glowplugs, cheap/simple fix... Also lost the IDM, got a performance upgraded one from Swamps Diesel WAY cheaper than the Ford house..
When shopping for a 7.3L truck, some things to pay attemtion to are cold start willingness, look for fuel leaks in the "V", Check to see if the fuel heater wire is disconnected from the fuel filter base... on the glowplugs, there is a simple v/m test that will determine if they need replacement or not.
Fuel leaks in the "V" arent really bad, jsut a couple hours of work to repair and may be used as a bargaining tool on price...
The fuel heater is a KNOWN source of electrical shorts that will disable the engine... having the heater wire unhooked is ok if you live in a mild climate...