If you are a one man outfit and intend doing all the windows with WFP (not just the upstairs) then 350l should last you a full day....once you are experienced that is!!
To begin with you will use far more water than you actually need to, but you also need to use lots of water to begin with as you need to learn your technique.
I have a full van mount system, I've also a cheap backpack that I've sort of converted to a trolley system
To be honest I see no point in paying big bucks for a trolley system.
If you have a tank in your van, a submersible pump and a twin DI system - as I understand it, the water up in parts of Scotland has a very low tds - then you should be able to get up and running quite cheaply, especially if you don't need to go the RO route.
You could use a backpack, stick it on a shopping trolley (sat on a larger battery to allow you to work all day) and away you go
Going back to how much water you might use; With a 15l backpack I can do 2 standard sized semi's, Tosh- who has the 18l shurflow backpack - can't manage that with his system.
I think it is Jeff Brimble who uses way less than either me or Tosh, it all depends on your method/technique.
At one point, on a busy day, I could go through 650l of water with no problem at all, now 650l will last 2 of us a couple of days.
If your tap water TDS in your neck of the woods is low enough, and you can go the DI route, then topping up your van tank through the day shouldn't be a problem, so even if you do use shed loads of water you will still be ok.
Ian