Two things you could do...
1. There is FreeNas
which you could explore. Its a network file sharing OS, all you need is a machine to install it to... or it runs off a USB key too. Check the documentation on that though.
It can be interfaced on the pc, or remotely from a browser which would make your administering of it a bit easier.
2. If you have a spare or unused copy of XP, you could install that on another machine and connect it to the router.
Then just set up as many folders and shares as you'd like. You could also use internal drives.
I think this may be the easier option.
Either way, it requires a low-end pc. And once you get it up and running, you wouldnt need the keyboard or monitor. It'l just need to sit quietly somewhere in the corner.