I would stay away from the 6 leaker I mean 6.0 liter. I'm a ford guy but won't even look at diesel ford trucks made between the years 2003-2010. The 6.0 and 6.4 navistar motors are the reason why ford is making their own diesel motors.
The 6.0 has a problem mainly with the EGTs (exhaust gas temps) so if you run the truck hard your going to have problems with the motor.
I would look for a truck with a duramax diesel or 7.3 diesel.