Alright, I decided to chime in with my unwanted 2 cents anyway--you did title your post "Best Puller".
THIS is an example of what I would get. You can get them cheaper.
The 6.2/6.5 has been in production for 25 years, and still is. It's proven. It survived the initial morons that didn't know about diesels and in it's quarter of a century of production it has certainly shown its ass a time or two but the bottom line is, it's a great motor and extremely efficient.
Will it beat a Cummins or Powerstroke in a footrace?
Nope. Everyone knows that. It's an old indirect injection motor, mechanical, low pressure injection, high compression limits boost and it isn't built for the torque the newer motors can make.
But, it will respond well to 12psi of boost and the injection pump can make 250hp/450ft# on that 12psi. If you monitor the EGT it's no problem. It's an extremely simple motor that any idiot can work on, and unlike any other diesel I've stuck my head under--you can actually see it in the engine compartment.
That newer body style truck with a manual overdrive or a lockup converter can get 25+ mpg on the highway at 70+ mph. I had a '90 2500 and got that. I also made 16mpg pulling my B2 on a trailer at 70mph. No problem. 450ft# gets you over any hill without a downshift unless you get into 7% territory. In my current truck with 4.10 gears, I've had to downshift once for a hill--on the Mountain Parkway in Kentucky--it was 7 miles of 7% and at the very end it suddenly rose steeply and the truck shifted itself.
I don't know what a 600ft# Cummins gets you--you don't get a downshift on the last 1/4 mile of that particular road I suppose. I've towed everywhere from Colorado to Taxechuessets and Florida to Michigan with this truck. It's a 250hp motor and if I am racing another diesel, which I have, it stands its own. It's a very quick truck empty--nobody can beat you across an intersection with that 21.3:1 compression.
And I paid $5,000 for this truck--a Forest Service surplus truck with 47,000 on it. I paid $900 for a used Banks turbo kit on Ebay and then $2,100 for the overdrive unit. It has about 84,000 on it now and I replaced the waterpump and the mastercylinder and done 2,500 mile oilchanges in the 40,000 I've owned it.
It's the best vehicle I've owned and my second 6.2 diesel. No you don't get the cool points of having a big Cummins C but you aren't going to make as many payments, it's easier on fuel and it still goes the speed limit + 10mph easily with 9,500#--the heaviest I've pulled with it.
Don't think you'll be in the slow lane watching people go by. A 450ft# truck beats any gasser when you're on the interstate loafing past them at 2,00rpm on hills when they have downshifted twice--it's a beast of a motor with a turbo on it.
But I still wish I didn't have to have it. I want a turbo-diesel Jetta.