Wouldn't that data be encrypted? And wouldn't it be stored redundantly?
Yes and Yes.
All data is encrypted without any way around it. The host cannot get into the files and see what they are. Redundancy is also a mandatory, once you upload data its stored 30 times over to ensure your files aren't lost.
https://forum.sia.tech/topic/108/how-sia-worksEDIT:
With that being said, I still wouldn't use SIA for storage just yet. There are a lot of things missing from a user perspective and even after these things are added, SIA's real vision is not for regular users like us. They want 3rd parties to use SIA's API and handle all the customers for them.