The 6.0 Powerstroke through 2005 model was the Achilles heel for Ford. That is why it is so cheap. Starting in 2006 there was a little improvement in reliability but not much. Some were very good, MOST were garage queens that Ford paid tens of thousands of dollars to repair and they still were not reliable.
On the other hand, the 7.3 was the best diesel Ford ever installed in their trucks. Other than being old school technology, it is still an awesome engine and will run forever. The big problem in that era was the transmission, but a $2000 rebuild with HD upgrade parts will make that a non issue.