Stepvan
That truck looks like a Chevy, but the body might be utilimaster. I don't think Grumman. As for the running gear, they are usually van chassis, so E-350 if a Ford.
I have a Ford E-350 chassis under a Utilimaster stepvan body. It is all aluminum, so there is no corrosion issue in the body, but the frame is another story.
The International stepvans are way sweet, and can often have a very heavy chassis, indeed. 2-1/2 ton chassis are pretty common, and I've seen them over 5-tons.
I'm in the process of replacing mine with a trailer, because mine is pretty small, but in colder climes, they are very handy.
I'll warn you now, though: They are not easy on fuel. I get 10 on good days, and mine is a fuel injected 4.9 liter Ford straight six. Wind is your enemy.
In our work, I'd say get a big one, with a diesel, and a dually for sure. The dually set-up is more stable in wind, and while parked, and the deisel pull-power has been wished for many days in my little rig.