I just purchased a 2005 6.0 excursion. This will be the second 6.0 that I own, the first being a 2004 F550 dump truck that has over 200k on it with no problems. Before purchasing the excursion I did as much research on the vehicle and became very good friends with my local service rep. He ran an oasis report for every vehicle I found. The report can only tell you so much though, so you still need to do your homework by talking to owner of the vehicle. What I found out was most guys that have these engines and have had problems with them and gotten those problems fixed were eager to tell you that they have corrected the problems or have done the upgrades anyhow to prevent failures.
I would also try to stay away from the ones that have had a lot of modifications done to them such as tuners and different turbos, might be a sign that they were driven hard.
There are good ones and bad ones out there, as other forums have suggested i would invest in a scan gauge 2 and check for codes and other things that might tell you up front if the engine is in need of some work.
Other than that I love my excursion and wouldn't trade it for anything. Happy hunting.