The amount of water that you plan to change sounds fine. most people change about 20-25% per month. As far as an RO/DI unit most of the units are the same except for the filters themselves. I purchased a unit from costco, then replaced the membranes after a couple of months with the kent cartridges. The unit from costco cost about $175, and was fairly easy to install, as I am not the DIY type of person.