Im new to wfp, i found the cheapest way to start is to buy a backpack £175, cheap trolley (b&q £12) and some water barrells.
Re wire the back pack so you can plug it into the van (running off the van battery or the trolley).
Plus you can still use it as a backpack.
This isnt the best way to setup a full system but its the cheapest way of having a backpack, trolley and be able to plug it in the van, its got a 80psi pump.
Its cost me:
£175 backpack
£15 trolley
£35 switches, plugs, wire (maplin)
£75 RO unit
£55 DI
£40 water barrells
£250 on two poles 25ft and a smaller one.
hope this helps