id say forget it mate.
either all or nothing.
messing about with barrels and filling up backpacks etc will be time consuming.you d be quicker carrying on 100% trad IMO.
if you want to start small then a 250L tank and 100m of microbore would do you for upper windows only.i had the idea that id just do a bit of wfp and trad the rest but you soon realise that its easier(and quicker)to wfp nearly all work.top and bottom windows.
i very rarely get complaints about cleaning wfp.it does an amazing job.much better than mop and squeegee 95% of the time(frames etc).
in fact i would just go with this
500L TANK and frame £400
pumpbox and battery £400
SLX 25 and brushes £300
claber reel and 100m microbore £100
various hoses and connectors,etc £100
DI ONLY(if water is below 100tds)and resin £200
not sure about an RO as ive never used one.
so for around £1500 you could be up and running with a good wfp system.min payload for a tank of this size would be 900kg but you could get a smaller tank to be within the payload of your van(max 300L in an astravan IMO).
once you start wfp you ll soon want to use it for most work as its such a handy cleaning tool!!