Good idea on wrapping the tank in black plastic. The tank will quickly turn green inside with sunlight. They make larger tanks made to install underground out of dark colored PVC. I believe there are tank heaters available, although your plan to store inside is more practical. Drip irrigation suppliers have a lot of specialty items for what you are doing. Condensate water is slightly acidic, our public utility won't allow it to be discharged down sewer lines if you bother to ask.