I had a 08 6.4, the truck was deleted, once that is done it will run great. They did have some with radiator issues, but by now I would think those would be mostly resolved.
I never owned a 6.0 but have a friend with a diesel performance shop, he can bullet proof one for under $4k, so if it hasn't been done I would plan for it.
For that size trailer and trying to keep on a budget like I can imagine he might be on, I would look for a gas engine truck, gm 6.0 or ford v10. Maybe even consider a 1/2 ton, I cant imagine a 21' weighting that much as most 1/2 tons can now tow 10k lbs.