My city's water is super hard too, and @keith943 is right, it makes a huge difference. I started using filtered water last year and my coffee got way better.

That sounds about right for some of those old houses. You gotta sister new joists right alongside the old ones to get a solid base. Pressure treated lumber is your friend for that, especially with the moisture you find. Then you can lay down a proper plywood subfloor before anything else. It's a ton of extra work but the only way to make it right.