first thing you need to decide is if you want RO water on demand....
If so then you will want a system with a captive air tank (aka bladder tank, aka accumulator, etc)
Then you need to decide on how pure you want your water to be.... under ideal conditions a RO will reduce your TDS by 98%, Realistically, this will probably be more like 95-96%
So if you have 200 ppm tap, the RO water will be 10ppm, if you have 1000 ppm tap the RO output will be 50ppm, etc
To remove what the RO membrane doesn't we use DI beads....
Also, if your supply cannot maintain 45-50psig minimum, forget about buying a RO unless you install a pressure booster pump.