I have very low water pressure, so I added the automatic booster pump ($80) and the float valve ($25) to my garbage can reservoir. Free shipping and a TDS meter are included. I'm very happy and my RO water is 1ppm and final is 0 ppm.
That depends on your tap water quality. My tap water is 19ppm TDS, the same as my water going INTO the RO filter. I suspect I may never have to change them. But if your tap water is 300ppm you will have to change them periodically.