Buy the cheapest PC you can find and add a new large hard drive to it. $150-300 for the PC, $100 more for hard drive. Share a monitor with another computer - once it's all working, remove the monitor and leave the server running "headless."
My preference would be for linux and Samba (SMB = server message block on Windows) as the operating system and sharing method. It's easy.
But - Windows 2000 or XP or Vista should also work, if that is already installed and working.
Software for syncing can be freeware on the Mac. Not sure about freeware on the PC.