Since you are not opposed to multiple tanks, why not put some in the greenhouse and some in the ground? I would build an in-ground cistern for drinking water, of either parraged block or solid concrete so the lime creates an unfriendly environment for bacteria. It is how most people in Bermuda get their drinking water. If you use block, do some research as they frequently develop cracks and leak, causing owners to install a liner which defeats the purpose of the block. I read once of a technique where little or no mortar is used between block, and then porraged with a skim coat (like stucco) to waterproof, which is supposed to be more stable and less suseptable to cracking than using mortar. Steel reinforced concrete would be best, but most expensive.
Never get into a hole deeper than 4 feet without something in place to hold back the dirt. Hole collapses kill a lot of people.
I read an article about building a solar hot water storage tank the guy built out of 2x4 lumber, 3/4 ply, and a pond liner for about $200. It was about 4x4x4 (450 gallons), if I remember. If I find the link I'll post it later.
Around here (east coast of USA) you can buy plastic tanks about that size cheap, that were used for food grade soilbean oil or some such.
Btw, are you familiar with first flush rainwater collection? You could build a small tank of 50 gal (foodsafe 55 gallon drum for $20) to catch the rainwater, and when the tank fills it backs up the downspout and through a T into your drinking water tank. That way, all the bird poop, pollen, and dirt on your roof stays out of your drinking water. After the rain, you empty your first flush tank into your greenhouse water so it is empty and ready for the next rain. Smaller roofs would use smaller firstflush tanks.
Best of luck!