Wireless Network Syncing

Hello,

I currently have a W7 Desktop, MBP, MBA, and Mini. The mini is running as a server. I have a 3TB RAID1 external drive hooked up to an Airport Extreme right now to act as a NAS drive and backup.

I currently use SugarSync to sync my Documents folders on all my devices, however I am now using over 100GB of space (my work has to do with a lot of pictures from high speed cameras) and want to avoid paying too much.

I was wondering if it is possible for me to set up a syncing service between the Documents folders of all my devices somehow using the Mini + External Drive. Ideally sync only when on the same local wireless network to avoid security issues.

Please let me know if this viable; I am willing to purchase any necessary software.

Thanks,
Brian