The model I have says to replace the filter once a year, and it's $170 AUD. I imagine you could probably get away with using it longer than the OEM recommendation though.

I had the same thought about demineralised water, you can get more expensive models which remineralise the water after, but it looks like it's not actually that important health wise because you get the absolute vast majority of your minerals through food, not water. And remineralisation is mostly for taste rather than health. Though I don't find the demineralised water tastes bad, but if you're used to drinking hard water it might be different.