Hi Dream Clean,
If you have a backpack then presumably you have some sort of purification system to produce the pure water for it?
You shouldn't consider putting an IBC into a van - securing it will be difficult and unsatisfactory.
You need an RO (unless your tap water is 25-30 tds or less) and if your water pressure is less than about 60psi you need a booster pump.
Depending on the quality of water from the RO you will need a 5lt DI tank - smaller will not last long enough. You then need a tank in your van, and proper fittings to secure it, an 85ah (min) battery and some means of controlling the water supply to your brush - perhaps the backpack has a suitable arrangement. If the backpack pump is at least 60psi with a capacity of about 4lpm you could use that, mounted in your van.
Then you need a metal hosereel and 100mt of micro or minibore hose and fittings.
You're looking at about £800 to £1,000
You also need some way of getting the water from your IBC to your van tank - if you can mount your IBC up on a stand, you might be able to fill by gravity, otherwise you need a submersible pump, about £35 and a length of 1" hose at about £2-3 per metre.