Starting out I'd get a trolley and some barrels. No big deal if you decide it's not for you. Do you know the tds of your water? That will determine wether you need an RO system or double Di.
Old Croucher will be along soon to give a far more detailed answer. He's much better with words than me. Oh and he cleaned the windows on Noah's ark so he knows what he's on about. .
How much water do you expect to use a day or week ? Say you need 25 litres (may be first clean s )to Do a house and you expect to do 20
Houses a week to start you'd need 500 litres a week ? A small r o would suffice but ., you need to have a storeagentank as Ro
Units make water slowly and di is instant . For small r o units I found http://www.johnhewitt.com/ro.htm to be very good worth talking to him he will tell you what's best for your needs .
One teeny wee thing I could add is you could trad bottoms. It's a skill worth learning. Especially at the start when you will use a lot of pure on first cleans. Even if you go 100% WFP Mrs Smithers will probably want her insides doing at some point ( oh err missus) and gardiner poles are brilliant but not so good in a living room.
I got a full kit last year that came with RO, trolley system, pole and barrels. Xline systems will sort you out. I have just added a 350 litre pure water tank too as barrels became too much when you get larger jobs etc. Best of luck.