Since you have an interest in Linux I might throw out there Bluecherry. I've ran it for about a year now and it's been rock solid. I too didn't have interest in running Windows, so this fit the bill quite nicely as the server is Linux based. It also has a cross-platform client, so whether you're on Mac, Linux, or Windows you can directly connect to the server with a native application instead of some sort of RDP/remote connection to the box. Kind of a nice setup. No dedicated mobile app, so that's one downside, though you can connect to it for live view pretty easily with Tiny Cam, IP Cam Viewer, etc.
I've never done a real-time sync with my current setup, though there's an endless array of Linux utilities you can look into. One called lsyncd comes to mind, which I understand is a real time "whatever exists in source, sync to destination" kind of daemon. Typical utilities beyond that are also available -- rsync, cron, etc.
My server runs an i3-3220T, which is kind of laughable in the horsepower department. Even considering that my average load is next to zero (right now load average: 0.04, 0.03, 0.00). Running 7 cameras at the moment, 640x480/3 FPS and the remaining six are 3mpx/10 FPS.
30 day demo, so it's easy to give it a test drive. Just a consideration. Good luck in your search.