I've used both distilled and natural spring water and both worked fine. I do have a hanna digital ph probe that cost about 20 bucks on ebay. Probably something you need to get ASAP!!
As for buying water....I was going to get an RO system to hook into my water system but when I did the math, it was not even close. Buying bottled water for a grow is MUCH MUCH cheaper. It would take me almost 10 grows to equal the cost of an RO system and by that time I would be buying replacement filters and never break even. Wal mart sells 3 gal boxes for very very cheap.
I do not do hydro...just soil type grows.