Wow, 24'x9', what a great size. When I was shopping around for my greenhouse I actually saw some with gutters but they were pretty tiny. We wanted to do the same thing but settled on just running a hose from the collector barrel into the GH. Not great.
A cistern would be great. We had a cistern when I was growing up for laundry because we had sulpher water and it turned the clothes gray. When I lived in town I wanted a cistern for our garden but we never did get around to building it. I think you would be very happy with one.
Would be great if you could also use radiant heat floor from solar. I'm not sure if they even do that, but I know radiant heat is the most efficient.
