Nice system. The first drain and spillway are well thought out.
Not sure about elsewhere, but here in Europe, one can often get plastic IBC containers in a metal support cage (1 x 1 x 1 m cube shaped) really cheap. It's smart to examine the labels before buying, to make sure it didn't hold anything hazardous. They come with a tap too and store 1000 L of water.
So if one practises gardening, a bigger tank might be handy to have. Elevating the ground under the tank or digging a hole under the tap will ensure better access.