I think IPFS is an excellent foundation for such a system. Indeed, a few friends and I are working on it: [1], [2]. It's too early to call it robust yet and it needs an independent security audit, but it has all your bonus features:

* all storage nodes can't decrypt anything

* you can grant read and write access independently to individual files or folders

* you could easily store all previous versions of data, by never unpinning anything in IPFS, or having another server listening for updates and pinning them

[1] https://github.com/Peergos/Peergos

[2] https://peergos.github.io/book