Distilled or Regular Water

The biggest issue is far from calcium buildup or any of that. It's that with an aluminum radiator and cast iron block, you have just added (with tap water), an electrically conductive medium otherwise called an "electrolyte". Yes, you just turned your cooling system in to a battery comprised of two dis-similar metals. (Think: Copper and zinc, in a lemon, science experiment).

Pure water does not conduct electricity (the best for your cooling system is DI (Deionized) water from a laboratory, but that's going to extremes). It's the dissolved minerals and such in tap water that makes it conduct electricity. Think: this is why dropping the hair dryer in your bathtub while you're bathing is bad; the salts from your sweat make the water conductive.

Using distilled water (which is not ion-hungry, moreso than any other water) is the most cost-effective solution. Use a good coolant (Mopar G-05 is tops) to prevent rust and minimize corrosion. A bad system will actually have enough electrical current that you can measure the current in the cooling system. That current is corrosion happening. Change your coolant every 3 years, and I highly recommend a magnesium sacrificial anode. The Magnesium gets eaten more readily that your aluminum radiator, intake, heads, and timing cover. You'll have to replace it every so often.