We have pretty hard water here too - this is a good post though, I've always used distilled water, that's what my Gran told me to do. Filtered water is even easier, we have a 'fridge with water in the door.
My iron is mostly plastic except for the sole plate; it's a cordless so all the heating elements are in the base and not in the iron itself so I am not sure if metal corrosion is a worry with mine at all. The tank is definitely all plastic, it's translucent. I don't actually know what my manual said as far as what water to put into it...I didn't read the manual at all, I figured I knew what I was doing! Who knows where it is now, probably got recycled.