I'd just buy a di resin vessel to begin with . Plug it in to your customers tap and connect water fed pole. Conny roofs come up great with detergent or tfr etc. applied and washed with mains tap, final rinse with pure. If you were using it loads by all means then buy an ro, tank, pump , controller, reel etc. but why not test the water first ? pun intended
I use a 4040 membrane with two pre filters and di, 4 stage ?
I dont really know about the smaller systems and what a 3 stage consists of
filter, membrane, di ?
Remember 75gpd is probably US gallons and optimum conditions that nobody will ever come close to. yes it will purify water but so slowly you really wouldn't want to bother with it. Its the sort of thing aquarium hobbyists use.