Sad thing, I have been looking for a "cheap" F250/F350 as well... no such bird around where I live, because with all the housing construction, any 3/4 to 1 ton truck is in high demand.
Even though I like Ford, I probably would point you to a Dodge than recommend a 6.0. I personally am looking at the V-10 or the 6.2L gasser, even though it has half the torque of the diesel, just because it has far fewer parts and simpler to maintain for the most part.
Is the 6.0 + bulletproofing a decent engine? Yes. However, unless the added components were factored in the purchase price, another brand/year of diesel is probably better, price-wise, if one doesn't go for a newer model with a 6.7.