There are 4 factors that you need to consider before buying a water filter system.
1-Water Pressure: Most RO units need at least 45 psi to operate correctly. If your pressure is lower than this, you might look for a RO with a Booster pump similar to this Bluonics 5-stage RO with a booster pump.
2-Filtration Quality: The quality of RO filters can be measured by the total dissolved solids (TDS) of the filtered water against the source water. A regular RO removes up to 99% of over 1000 contaminants, including lead, chlorine, fluoride, arsenic, asbestos, calcium, sodium, nitrates, and more.
3-Flow Rate: Reverse Osmosis has a gallon per day (GPD) number that refers to the amount of water a unit can filter per day at an optimum condition. In most cases, the amount is about 10-20% lesser as stated.
4-Budget: Before purchasing a RO system, consider both the initial cost to purchase a system and maintenance costs to replace RO filters.