No freaking way! Rental, auction, on lot almost 5 months? Carfax does not mean much per my insurance agent. Unless its work preformed at a dealership, it probably will not be on there.. No one else reports out.
I just bought a 2013 GMC Denali HD (2500) Crew Cab, Z71, 6.0L gas, 100% loaded everything with just a little more mileage for about the same price. This was a one owner, north Louisiana truck. Looks like it had never been down a dirt road (inside frame rails clean), you could eat off the engine, and the receiver hitch still has paint inside (never used, never towed). It was just an oversized car for the previous owner. It even still smells new. I did not believe it was a 13 when I viewed it and test drove it.
With this said, I looked at trucks for about 2 months. I have seen many listings and prices are wild. I'd look at another dealer. Some can make deals, some cannot. I'd run away from this deal.
You live in a FLAT area. For what you do, a gaser would be more than sufficient and cheaper to maintain in the long run. The GMC 2500 is 9900 GVRW, 13700 Towing 5th/Goose, 13000 Towing conventional. The 6.0L gas is 360HP/380TQ with a pretty flat torque curve due to VVT. Personally I don't see a need for a diesel for what loads you describe.
I am partial to GM products. Dodge/RAM was my 2nd choice with the 6.4L gas. But read some mixed reviews, it's still new. 5.7 "Hemi" wasn't even a thought. It's not a truck engine. Its a passenger engine for cars, vans, SUVS, and 1/2 ton trucks.
My $ 0.02
CT