It is a great thought to use a centralized purification system for all the five families residing in your area. This will not only reduce the individual cost but also will be more convenient.
You can ask for any government subsidies for water treatment in your locality. For this you may need to approach municipality office in your area. they will guide and give you a list of companies who have pact with the government for such subsidies.
In the mean time, you can also contact various purifier manufacturing companies on their official website or approach their near-by agency.
Eureka Forbes is a big name in this field who claims to test the available water and accordingly suggest the type of purification system which you will need.
RO (Reverse Osmosis), UV (Ultra Violet rays used to kill germs), RF (Radio frequency used for purification)are the major techniques used.
Till then, to remove hardness and salty taste of water, boil it and filter it in normal candle-type water filters. Boiling is still the most effective method to kill the germs.
Drink clean water and stay fit.