If you want NAS only, and are planning to spend money on it, then why do you want to re-use a desktop PC?
A small form (credit card sized) arm board, (cubie board, Olimex Lime, raspberry pi, bananna pi etc) boards are probably going to wind up being the same cost by the time you're done buying bits for the PC! (but smaller quieter and require less power.)
I'm not sure I get why you want a NAS OS, to share disks rather than using either windows or Linux that would enable to to put the box to more use (e.g. a media center, game server, web server remote access VPN endpoint etc.)