The price is a lot lower if you buy it in 55 gallon drums (~50%), but you'd want to split it with several other van builders because you'll have way too much.
Another option to consider is pour-in-place polyurethane, which you can install in a more controlled way without a spray gun. (That's how fridges and freezers are made, i.e. a slow-expanding isocyanate/polyol mixture is poured between the outer walls and the liner.) And if you line the inner wall with 0.001" aluminum foil, that shold stop the diffusion of the blowing-gas and prevent ageing.
One thing to watch out for is that the thermal expansion coefficient of the foam is 10x that of steel and 5x that of aluminum. NASA has some technical papers on how they modelled and managed that with the Space Shuttle external fuel tank, and I think it wouldn't be a problem for most people. (In the 15' bay of my aluminum stepvan, I'd get ~0.5" of length mismatch when the temperature shifts from -40F to 120F.)