When I first started I had a Discovery in which I carried six 25litre drums and a trolley system. Roof rack for the poles and away I went. Worked good for a while, then I realised how much diesel the disco was using. So... Been out and bought a very cheap Vauxhall combo van. Seeing as how I'm a mechanic anyway I thought it wouldn't pose much of a problem to fix it up. It flew through it's MOT even if it does have 187000 miles on it.
So been looking around and got a few idea's about van mounted systems. Went to B&Q and got a 190 litre lift tank. A pump, some fittings, etc. I've made a subsantial frame which is securely bolted through the chassis with plates that will not 'pull through' in the unlikely event of any van/hedge interface problems.
The system I have is working well I can report. The best innovation is the purchase of a remote key fob device from maplins. Wired into the system and the aerial wired to the now disconnected heated rear window of the van gives me remote on off around the house I'm cleaning.

Even works when not in direct line of sight with van. Have thought about the three channel one so I would have three speeds, but am managing with just one speed up to 3 storeys.
Only issue so far is a certain amount of surge due to the tank not being baffled when cornering, but also being an ex-sailor i do not suffer from sea sickness.
All in all, if you really want a van system, you can do it a damn sight cheaper than advertised. Heath Robinson maybe, but to me that is half the fun.