I have a system that uses a series of 6 55 gallon plastic drums (they were lactic acid barrels from a local cheese company). I rigged them up using PVC fittings and pipe dope/cement to build them in series of 2 tanks connected at the bottom, then a spigot on each bank of 2. They are all connected at the top, so when the rain (from my downspout) overfills the first 2, it dumps over into the next 2, and then into the last 2. This way, if something fails, you only loose 2 tanks, and are still operational.
Note-- that is one hell of a lot of weight, and since it's under gravity feed, I had a freind build a steel rack behind my garage to keep them about 3' off the ground.
Also, The "bung" on the tanks is mounted on the bottom, so you can get rid of leaves and gunk if it collects there. It works like a dream. I usually have at least one of the banks of 2 with an old pillow case of manure for occasional feeding of the plants (manure tea).
Just a thought.
John
ps, yeah, my best friend is an engineer, and yes, my fiance and neighbors think I lost my mind!