You will be fine with 1 di.You dont need ro.
When doing first cleans you will use loads of water,what i do is use tap water from customers tap if i can,or bypass di in van and use tap water from my tank,then give the frames a really good clean and plenty of rinsing,they will come up really good,then i do a few houses nearby and come back an hour or two later when there about dry and give them a very quick clean from the top of glass down with pure water.If you use just pure on first cleans you will go through LOADS of resin.My tap water is about 53/54 and i use twin di in van to save on resin,when you get more customers you can add another di,11 litre will do.Dougie Daqua sells good vesells and the best resin(tulsion mb115).
I bought 6 rectus 21 couplings from Gardeners over two years ago and still have 4 left,never seem to get leaks.