I personally think that FOWLR do not need 0 TDS water from RODI or DI unit. Just tap water is fine. But then again, if you can its definitely better.
With you tank size you'll need some big and solid skimmer, Don't think even the BM Curve 9 will make a good choice since you have over 1200L water. Deltec 1500IX is a good choice IMO.
Such a big tank is also gonna be a challenge to chill, FOWLR ideally should have a chiller running as well.
Edit: Don't you already have the FOWLR set up?