Hello, I am an environmental engineer sharing my design for a ~300 gallon IBC tote rainwater collection system. What makes this different and beneficial for self-sufficiency is the low cost per gallon. This tote also provides a large enough storage tank to use a dedicated irrigation pump for reuse. You could make this as simple or as complex as you wanted to.
IBC totes are frequently used for storing and transporting food grade materials, so there is an aftermarket for cheap used totes, you just have to verify what they were used for. I got this 275 gallon tote off of marketplace for $100 USD. It was previously used to transport food dye. They are highly durable because they're built out of HDPE and galvanized steel.
Other Materials:
Leaf filter, from $10 to $70 - my preference is the Leaf Eater Advanced ($66)
Downspout diverter, my preference is Earth minded Flexifit diverter ($20)
PVC pipe and valves, figure $50
Total entry cost including tank: $180 - $240
Optional:
Lid adapter for two inlet ports $10
1 HP irrigation pump $140
Discharge splitter $10
Wi-Fi controller $30
Piggy-back Level float, alternative controller $50
Ozone injector to keep the water fresh and odor free: $400 (luxury) JED Engineering model 203
Our UK garden is about 75x75m, which is a decent size, but not massive. About half to 3/4 of it is grass, which I hardly ever water, and haven't this year even though we had a drought.
We have 5 IBCs, and several other randomly-sized water butts (~7000l total (1500gal UK, 1850gal US)), to keep our veggies and fruit bushes watered. We ran out this summer, and had to use about 500l of tap water until it rained again :(
Dry periods are likely to get worse in future, so I need to get more water storage. I also want to start using rain water to wash clothes (or at least fabrics we use for our pet rabbit houses), but I need a good filtering system first.
We have 2 IBCs stacked, and I was trying to use the top one to gravity-feed our downstairs toilet cistern for flushing and worked well, although it took 7 minutes for the cistern to fill, which isn't great if you have to accommodate a lot of people. It is good enough for us though.
I could add a pump to speed up the process, but I'm trying not to add more cost and complications to the system unnecessarily.
I can fit 1 more IBC in somewhere - they don't look pretty, but they are very practical. I normally transport them home on the top of my car on a roof-rack.... and drive slowly! :)