Yep, distilled water is best. The less stuff in the water besides water the less stuff there is to react with and corrode.
Don't use a radiator flush chemical, just water. My nephew used to use those flush chemicals on every new/used car he got. A couple of months later he was replacing leaking freeze plugs. When he stopped using the chemical, he stopped losing them.
we have moderately hard was here, and I must admit I have always just stuck the hose on it and filled and flushed that way. Then again due to lots of very hot weather I have my radiator rodded every 25,000 k's anyway......
My well water is darned near sea water. (The valley I live in was once a salt-water sound, and the ground water under it partly sea water. Think of Washington's Puget Sound, then add a quarter million years.) I use jugs of distilled water I buy at the store for my radiators. It's cheap. Using it is also cheaper than repairing salt corrosion in the cooling system.
I flushed the cooling system, all the coolant that came out was clean. I refilled with water and took for a 100+ km drive with the heater on to flush the system and again the water was clean, no rust or sludge. I did the final refil with a 50/50 mix of Nulon Long Life Concentrate Coolant and demineralised water.
How often should I service the cooling system? The bottle of coolant says it lasts 5 years or 250 000 kms, which ever comes first but that is probably far too long?