If your water comes from a well, it probably has some bacteria in it. However, for most people, their immune system can handle a small amount of bacteria. If you are drinking the water now and merely want purer water, you could install a regular reverse osmosis system under your sink or in your basement. This will screen out bacteria, but if the membrane ever fails, bacteria could get through. As an extra safeguard, you could install an ultraviolet light.
Secondly, if your water has a normal amount of Total Dissolved Solids (i.e., 500 or less), you need at least 40 p.s.i. of water pressure. If your shower water seems normal (when compared to hotels, your previous homes, your friends' homes, etc.), you probably have sufficient pressure. If you are doubtful about your pressure, you can buy a water pressure gauge at your local hardware store and measure the pressure. If you don't have enough pressure, most well pumps have a pressure switch which can be adjusted to increase the lower level of the pressure range. Otherwise, you can buy a Booster Pump for your R.O. system, a Pressuriser for your whole house, or possibly just run your R.O. system without a tank (the bladder in a tank creates back-pressure which has to be overcome). In any situation where you are unsure of your water pressure, it is important to periodically test your water for Total Dissolved Solids. This will give you an indication of how well your R.O. system is working.
One of the functions of a carbon pre-filter is to screen out chlorine. Because of this, some dealers don't sell a carbon pre-filter for well water applications. However, another important function of a carbon pre-filter is to screen out organic chemicals and to serve as a "safety net" for what the sediment pre-filter fails to catch. The best way to preserve the life of the membrane (which is the big ticket item in your system) is to do a good job at cleaning your water before it hits the membrane. Thus, we recommend the carbon pre-filter even on well applications.
If you would like an idea of the kinds of contaminants which an R.O. system can remove from water, please refer to this page.