Water Storage: Build a pond, cistern, or just buy a water tank?

I need to store roughly ~30,000L of water over a 6 month period in my semi-arid homestead. I can fill that in the winter season when it rains, from the roof and other structures.

What kind of water storage solutions have you guys done? I was thinking of building a smaller pond (~2m x 3.5m x 5m), but I think evaporation will be a big problem. So perhaps build a cistern of similar size, but then I need to figure out how to water proof the bricks/cement etc.

The simplest idea, but most expensive, is just to buy 3x 10,000L underground tanks at ~4000euros each.

Thoughts?