Shoot for a minimum of three stages. Carbon, Sediment and RO
membrane. Filters with more than three stages will offer you enhanced filtration but the bulk of the work is done by three stages.
filters are sized by the amount of water they can filter per day. The more common capacities are 25,50,75 and 100GPD models.
Ebay is a good source for cheap RO
units but I would also suggest looking at some other quality products like those by our sponsor AquaFX. The higher end units often come with TDS
meters inline so you know when your membranes are starting to slip as well as pressure guages so you know the inlet pressure. RO
units operate effecently at a specific inlet pressure range and if your to far below that then your GPD output will be affected.