this is the point
get a trolley system , try it out, if you dont get on with it (for some strange reason) then if you have built it yourself, you have wasted 500 - 600 quid, not a major issue
than if you need a bigger system in a van, buy a tank and away you go, you wil stil use the same pump, poles, brushs, hoses etc etc, just change the 25 L barrels for a tank