Diesel life span

OP sounds like you want a diesel and are trying to justify it through costs. Looking at new you can never justify the cost, the diesel option, in most states diesel is as high as premium and when towing still around the same MPG, it won't add up. I ran it once and it came to like 300k miles for a break even. However if you plan to compare to buy a today's new 250/2500 with those small blocks I would go diesel anyway. I wouldn't even consider a small block for heavy towing.

My last gasser pickup went 360k before I traded for what I have now