Definately need an RO system. A sediment filter will not lower your PPM much if any at all. THis one isnt bad:
http://wegrowhydro.com/store/product.php?productid=7728&cat=279&page=3
Even at 3g a day, thats 42g in 2 weeks (typical res change over interval)That will work if you need 3-5 gallons a day. Maybe 10. But in reality it is for drinking water. Not to fill a reservoir.
What do you mean? Is RO water better for soil then well or tap water? Absolutely.does this apply to growing with soil as well?
is this what u havecheck out www.purewaterclub.com