Hi Michelle,
I had the same problem out where I am with hard water although we never had problems with rust- you just feel it on your skin and hair when showering. I called a few places and found our best option was to rent one, we could not afford the initial cost of purchasing one. After a couple of years they give you the option to buy it out or continue on the renting. We have just continued to rent it since they will come out and service it as needed if there is ever a problem and replace it even if necessary.
We do have to make sure that there is always salt in the thing, usually about 2 bags of salt a month. But even with the salt we do notice a residue/spots on our sink and tubs... not sure why, maybe our soap?!?!? But I just use vinegar to clean that. For our toilets I do vinegar and baking soda....I have never had problems with rust so not sure what to suggest for you for that, maybe try the same for that as well?
Good luck and blessings to you!