We have a well that collects spring water, I think unless you have a spring you may find that you are liable to pay abstraction charges. Our water is pumped uphill to a storage tank, from here water is gravity fed to the house, yard and fields, the neighbour also takes their water supply from our tank. As I understand you do not need to have your water routinely tested if it is only supplying your own property, ours now has to be routinely tested by the local authority at our expense as the neighbour takes their supply from the same source (free of charge although they are required to contribute to maintenance costs and electricity for the pump)
Last time pump replaced it was in the region of £800 or so, I think it was probably one of the contractors mentioned by Uponeacre as we are on the Powys borders.
You could probably ask the Local Authority to carry out the testing, I think it is around £200, although they tested both the tank and the tap water, the kitchen tap water goes through a UV filter. However the test did not really seem very specific, ie we were told that there were coliforms present probably due to plant roots that the water passes through to be collected, not much you can do about that, but it didn't confirm that or whether there were more harmful ecoli present etc. Just told us we needed to replace the filter/bulb on the purifier and to only drink boiled water - not sure we really need to regularly pay £200 to be told that and to do what we already do!! So it would probably be worthwhile checking how specific your water testing would be for pathogens etc with testing labs beforehand, they should then also be able to advise you on the type of water purifier you would need to treat the water.
Good luck