What kind of water you use really depends on where you live and how your city treat the water.
I use RO because I live in chandler/Phoenix AZ. I am about 8 miles away from the phoenix water area. Their tap water is 600ppm on the best of days.
Here in Chandler is usually 400ppm from the tap.
For me, it is only RO. I don't have to use a DI unit. It's not necessary to get my levels at 0.
I payed $220 at Lowe's for a 3 step RO unit and it gets my water at 0 and keeps it there for over 1 year with the same filters. (We also use ours for ice and drinking too) The kit included a holding tank, a spout, all plumbing pieces, and of course the filter unit itself.
Replacement filters are the cheapest at 20 bucks for my model. Other units were 50+ dollars a filter.
If your water is bad, even with a FOWLR tank, you will have issues with algae, diatoms, even cyano.
If it works for you with the tap from your area, then just use it. I think it is best like a trial and error thing, because your area is not always the same as someone elses.
Good luck!