first you need to find out the tds of your tap water.if its less than a 100(mines 35)then it would probably be better to go DI only for the hassle free way of purifying your water.
if its higher then i cant comment on the RO side of things as ive never needed to use one.bear in mind it takes a lot longer to purify water the RO way.
i use a 50L TROLLEY SYSTEM from pure freedom that suits me fine for big chunks of my work and it also doubles as a mini-van mount for stand alone jobs.i use around 250-300L day.(10-12 BARRELS).thats cleaning most properties wfp with the odd bit of trad here and there.
you need a 22ft CLX for regular domestics IMO as you dont want to give yourself an injury esp if you ve no experience of pole work.12 ft is too short.a 22ft will allow you to reach over most connys and flat roofs with ease.supalite brush on the end and you ll be away!!

every evening when i get in from the days work i run hose down the stairs from my bath tap(im in a first floor flat)through a monster 25L DI VESSEL and straight into my barrels in the back of my van.i have a tds meter to check the waters 000.it takes around 45-50 mins to fill up ready for the next day.
after 10 months WFP im like a well oiled machine and its by far the best thing ive done for my business!!if you ve got a lot of compact work with parking issues a trolley would be fine but i would get a 50L or you ll be filling up twice as much!!
if on the other hand you have lots of scattered work with lots of driving about then a trolley would be a nightmare!better off going straight for a van mount.
the waters purified the day before through the DI not as you go on the trolley.carrying a di with you everywhere is just wasting time IMO.you want to be ready to rumble the next day.WINDOW CLEANING!!