Rainwater Systems, Inc. was the first company I'd talked to about a potable rainwater system that didn't give me an "all or nothing" response. 

I was unable to afford the ideally recommended system all at once. They were willing to work in phases, as I could afford it. The rainwater system was integrated into an existing system that allowed me to have multiple options: all rainwater, all original system, or a split with both systems operating simultaneously, each in different areas. I started with a single 3,000 gallon poly tank. Later, I added another 3,000 gallon tank. Finally I was able to afford an "optimal" tank. They credited me for most of the original purchase price on the two poly tanks when they installed my current metal tank, with a liner. In addition, since the metal tank is located on a slope, they installed a very impressive retaining wall on the down slope side. The water has a wonderfully sweet taste. My only regret is that I didn't do this years ago.