For my build I went with a low power setup, I tried to find the lowest power Pentium at a modest priced (I think I used the G3220T), the cheapest mobo w/ the most SATA ports, (A Biostar B85 w/ 5 sata ports), a cheap 32GB SSD (SanDisk), and a quiet case (Fractal Define Mini).
A buddy of mine gave me some enterprise drives for free so I've been using those, I need to get some other drives and have some redundancy but everything I care about on the server exists on two other computers. Losing the media I have on their would suck but it wouldn't be the end of the world.
I use it primarily for plex and file sharing, it runs Ubuntu server, I don't prefer it because I've had issues with its package management system especially if one of the repos of a particular software goes down and has no timeline of when it will be repaired (I'M LOOKING AT YOU PLEX). CentOS does a better job of ignoring/handling broken repos.