I agree that a van system looks better than a trailer system,its easier to reverse for a starter, (you also need to think about below freezing conditions)although some might disagree. There are probably in excess of 15 companies out there so dont rush. Try to get to the fed show at blackpool, there will be one or two traders there.
It also depends on your customer base, is it domestic commercial or mixed ? This will sort out how much water to carry as it is HEAVY.