In my opinion, offline servers are the best. Think of the security risk if you connect a server to the internet with FreeNas or the like. And then later when your version gets old and no longer works online, you have to worry about the new version having compatibility with your dated hardware.
I'd say build your server with whatever hardware parts that suit your needs (from any hardware generation), then use your OS of choice, and keep the server hidden away and offline. I'm the kind of person who, when upon finding the ideal technology, I keep it around forever and collect spares of the exact parts that I would need to replace. I don't see myself forgetting XP anytime soon.. 😀
Dedicated Windows 95 Aficionado for good reasons:
http://toastytech.com/evil/setup.html