The hard water is going to be a maj. issue. I was in the same situation , and the build up on the screen/ medium used to get the h2o into the air was a problem. I know it's old school, but boiling a gallon of water a day always worked for me.

The AprilAire is what I use.

I have a well too. My water is hard and acidic so I have a two stage water treatment system.
An Acid Neutralizer stage and a De-ionization Flter.

It's essentially just two big bubbles of heavy duty molded polymer one holds a charge of Zeolite the other ( upper one) holds a rechargeable load of either Calcium-Manganese or Limestone. I prefer the limestone as the Cal Mag made the water too soapy
This neutralizes the acidity in the water.
I charge it with about $40 of limestone every year or so.

The Zeolite needs to be recharged on an ionic level which uses a Salt Tank. It runs water into the salt tank then runs the salt water back through the Zeolite. This alters the molecular charge in the zeolite causing it to release the crap it's been filtering out of the water.
The zeolite releases the crap it gets washed away to the waste outlet and then rinses the salinity away too. There is some residual salinity which I address by unplugging the thing.
The zeolite is a de-ionizing filter that gets sub micron particulate out of the water. It produces really pure water.

I unplug it so that the zeolite gets really loaded up over a month or so. Then I plug it in and let it run and recharge for a week and unplug it again. This ( I like to think) reduces the amount of residual salinity in my water.
I haven't tried to test it yet but I sort of think that the sale doesn't get 100% rinsed away and I prefer not tpo be drinking it all the time.
If I don't unplug it the thing re charges every day.

I suppose I could test my theory by letting it run and boiling a few gallons of water away to see what'sleft.
That's tell me with some good precision how much salt is a ctually left in the water.

is not water vapor the principle by product of a gas furnace?

I have one of those whole house units on my central AC. It's buggy. Sometimes the valve sticks and floods the cellar. The filters become incredibly filthy and full of mold very quickly.

I've had experience with Aprilaire, they were great and don't actually need cleaning as much as you'd think (we have hard water too)

Dave
www.lighting4less.co.uk