I understand the technology behind the DOFF systems, low flow/pressure, superheated water. Apparently good for cleaning vegetation/algae etc..
BUT.. and its a big but.. like Chris has said, why not get a 21lpm (or something similar) and a couple of 21lpm hot boxes. OR, you would probably only need one, because you can use smaller nozzles, reduce the flow, to bring the water temp up. I only have a 15lpm boiler on my frame, and when i do gum/paint removal/graffiti removal, i use an 03 nozzle, and the water gets pretty damn hot.
You can effectively turn any machine, into something similar to the DOFF system (albeit not as well) by adjusting nozzle sizes, flow rates etc..
If you set it all up in your van the right way, you could have quite a compact system, all mounted nicely, with reels etc and you could plumb in or remove each boiler with quick releases, add more hot water when you needed it, or just run cold for driveways etc in the summer.
Whole lot would probably cost you £4000 with all the fitting and hose reels and then your time on top fitting it in the van.
I have 21lpm, with a 15lpm boiler, and i have never not been able to clean something. Though its handy to know that Jamie rents the thermatech systems, as im getting quite a few enquiries with access issues for stone cleaning.