I should note, If you plan on going the RO or RO/DI route, you'll want a storage vessel with a float valve installed.
I've got around $500 wrapped up in a RO/DI system and a 50G HDPE storage tank. My RO produces "75GPD" of clean, pure water. Which in actuality is closer to 40 GPD. I could install a pump to up the efficiency of the system but haven't felt the need for it.
The major downside of an RO system is it produces about 3 gallons of "waste" to every gallon of "pure" water. I have an additional storage tank that i keep this water in so i can water my plants etc.
And, as Bob said there, Definitely treat with Campden tablets if you're not going to fully treat your water. Chlorine/Chloramines can ruin a beer faster than ABinBev.
Then there's always the "going to the store to get filtered water" option. Personally I'd invest the $20 on a TDS(Total Dissolved Solids) meter if you're going to do this to make sure that you're actually getting pure water.
I brew with water starting at 0ppm but I would say if you're getting water in the single digits to around perhaps 20ppm than you're probably good. As a reference my tap water in Wisconsin comes out of the faucet around 400ppm of TDS. The maximum "safe drinking water" level is 500, although its not really regulated, as "TDS" is a kind of arbitrary number based off the electrical conductivity of the water.
I've got family in Chicago, and I've advocated that they all have their water tested by an independent lab as I'm aware there is a city wide issue of lead supply lines. (just do a quick google search and you'll find info on that)