The first year for 6.0 was 2003 and that year and the 2004 were the ones to stay away but they were good after that. I know alot of people that have good luck with the 6.0 later engines. I would not buy a 6.4 engine from what I have followed in the forums.
The real problem here is finding a shop that knows how to diagnose the 6.0 engine. Most shops don't have a clue. Alot of Ford dealers don't have a Powerstroke tech that can work on them with good results. But there are good 6.0 men out there and thats the problem. Finding them.
I only have 7.3 engines in my 3 trucks because I know them in my sleep. If I buy a 6.0 I would also learn more about them as well.
Saying all this I just bought a 1999 7.3 in a F350 drw cc with 157000 miles on it and after a little tlc it is a fantastic truck for low dollars. It had a bad torque converter and I did the work on it so it was a home run for me.
Here is excellent site to research the 6.0
Ford-trucks.com