Easy to use cloud backup with local file server?

Hi there; friends of mine run a small business with a local Windows file server and were looking for a backup solution.

I was going to move them to a cloud storage solution, except that they also need to maintain local copies because (1) their internet connection is unreliable, and (2) some of their files are very large, and they need local access to work with them reasonably well.

Alternatives I've thought of:

- Dropbox for teams syncing folders on their local PCs -- ideal, but expensive

- Use a cloud backup service on their Windows file server -- works fine, but isn't exactly turn key. Plus, eventually the Windows machine will melt down and need to be rebuilt.

- Cobble together something on my own using a NAS, rsync, and some remote storage from S3 -- lots of effort, and someone would have to maintain and support it

Does anyone out there have an easy to use solution that lets you setup a local, multi-user file server, transparently backs it up in realtime, and comes with consumer/SMB level support?