I use NAS4Free because their community was much nicer than FreeNAS. I think you can safely ignore all the ECC RAM stuff unless you are storing really important stuff on the server. Anecdotal, of course, but I've been running my NAS4Free server for over three years on an old Althlon 860 and 8GB of non-ECC RAM. Though, I use it as a file server, so Plex runs on another machine. If you're at all comfortable with Linux, that's probably the way to go. It will be more flexible than the NAS OS options, and the documentation will almost certainly be better if you run into a problem.