Ahhh, I do top off with well water that was ran through the softener in winter. (Our well is over 700 feet deep) Is there any way you can capture rainwater from you roof or something similar? Most places in Hawaii that is how they get all their potable water know I have seen filters / screens to add to a down spout to catch water that way. If you have a garden that might be a better way to go as well.

As far as what to check for, for me it was overall TDS not matter what it was I was told over 2000ppm was to high, not sure what else they actually tested for, I know our iron or tannen is really high as well.