Your tap water sounds like mine and like they both have said it will not work out for you with that high of PPM. Not sure you will be able to do this where you live but where I live I have access to RO water vending machines in shopping centers. I drive my truck and take a big 5 gallon plastic bottle and two "new/clean" trash cans and fill them up with the amount of RO water I need.

Cost me $1.25 per 5 gallons so not too bad and a lot better that buying big RO system.