I am in a similar situation. Rebuilt a 2002 chevy 2500 with the 6.0 gas engine. New cab corners, repainted the truck. Long and short of it, I didn't do anything to the engine. It has just under 200,000 on it. The last three summers that truck has been used real hard. I am happy with the engine, truck just doesn't have the towing capacity I need. You can get a 2010 chevy 2500HD with the 6.0 the 6L90 trans and 4:10 rear axle to tow 14,000 lbs.
I understand all the reasons you want a diesel. If I pulled a lot of weight regularly it would make financial sense. If I put 5,000 miles a year on it that would be a lot. But when I need the truck I want it to work.
Thing is I am not sure if Ford has a decent engine period, in a heavy duty truck, that was holding up in the mid 2000's they might. I am partial to chevy's 6.0 LQ4.
The other manufacturers(other than ford,chevy) don't have the physical numbers of used vehicles available, such that you can get parts. Not to mention the knowledge base that is available for the Ford's and chevy's.
To answer your question, based on your requirements I don't think any diesel is the way to go. So then it is gas. Then if it is gas, I just happen to like GM's 6.0 and the right mid 2000's chevy truck can get me the towing capacity to get me by for the short period of time I need to use it.