Roof runoff needs at least a basic filter to prevent crud from getting into your pump and borking it up.

You can store as much water as you have room and money for. It doesn't need to be too much more complicated than running a down spout into a container.

I hate to bring this up, but, you may want to at least know if it's legal to do this in the jurisdiction that you currently reside in. In some states, you can get locked up and fined. 🙄

If it is illegal, there are still ways to do it, you just have to change methods.