Advice for a static system (beginner in need of advice)

Hello everyone, I'm new to the forum so please here with me I'm in need of some advice.  I have been running a property maintenance buisness since January and i'm seriously interested in incorporating window cleaning into my services.

I have been reading the forums and gathering as much information as possible and i would like to build a static system where i could process pure water and then pump it to a delivery system in my van. I am interested in harvesting rain water from my roof into a water butt and polishing the water with two 20ltr di vessels and then into a larger IBC tank for storage.  Is this idea feasible as i am a novice and could really do with some help so any ideas would be appreciated, i can't afford a r/o system yet and i don't have enough work to justify it so for now i think di would be the best option?  I'm really eager to get into window cleaning but i don't huge budget so I'm purchasing things as they go. Is anyone familiar with this type of system? Thanks. 

I doubt you would be able to harvest enough water, are you in a hard or soft water area? It would probably be best initially to buy your pure water in until you have enough work. 

Thanks for a quick reply, the roof in question is an outbuilding/shelter and its a huge area so should supply enough water for me at the moment,  i live in the south and in a hard water area.

Polzn bladz had a vid on youtube of this

He has a few big water butts linked together and harvests off a metal roof

Seems to be enough for him but he just does tops wfp

If you're into DIY, you can build a simple RO system (one pre filter and one membrane) for about £50-60. Perhaps a bit more as you need a booster pump to push the rain water through the RO system.

Not fast (mine produces about 200l a day), but good for a start. Does need more water though..