I'm not sure if I imagine your intentions right. So here are just some thoughts.
I did not figure out how to "connect" the UI part of Sia-UI to other than local node. I just hadn't enough time to experiment. From what I've seen, it should be possible.
It would be difficult to separate the files by users. Every user could see all the uploaded files.
I haven't tried it yet, but the description of Sia integration with nextcloud looks promising (Nextcloud as UI for Sia).
As you will be operating one wallet, you'll have difficulties to track and divide the spent SC between the users...