at the lowes down here, the filters on the 3 stage units are duel 5 micron filters and one 1 micron for chlorine, if its the same, i would keep one 5 micron, one 1 micron, and make a stage of the ion resin exchange, sand looking stuff, wich has like 4 different colors of resin , which is for the deionization of your filtered water....that would just fine for making ro, the 5 micron gets the bigger particles, but most homes already have one of them on there water supply line, so you might want to look into that, if thats so then go with duel 1 micon filters, along with that sandy stuff wich i think cost around 50 dollars for a good replacement cartridge that would work great for you...but i also think you can get the normal resin deionization filter for around 30