If I were in the market for a diesel and I had 20k in hand I would....repeat...would consider a 6.0 Crew Cab Super Duty F350 Dually lariat.... But only if I could get the truck for 10-13k. I would then bulletproof the motor with the left over cash.
The 6.0 has a fantastic transmission and the 05-07 trucks were very comfy rides.
At minimum on a 6.0 I would do head studs, BPD EGR cooler, BPD Oil Cooler, coolant filter, clean or replace the turbo, find and replace the bad injectors, and add a OBDII monitoring system to keep check on the coolant and oil temps.
The Rams and the Chevys of the early to Mid 2000's had somewhat cramped back seats on the crew cabs. The Superduty was the clear winner for me due to the rear seat space.
The 6.0 diesel is a GREAT driving motor when it's running right. It's quieter than a 7.3 or a 5.9 but you still know you are driving a diesel.... Unlike today's diesel trucks that sound very quiet.. It's reasonably powerful and gets decent fuel economy.
Thanks!
Jeremiah