Definitely scope the purpose before part selection.
If it is just a file server then I don't even recommend building a PC. You can get off the shelf NAS for that, a lot of routers even support just plugging a drive into a USB port. And if you want to build the file server anyway, the OS won't need much space at all, a cheap 32GB drive would be more suitable, and then have the storage drive.
Or even a NUC with an ultrabook processor and external drives would be low-power and fast enough for things like PLEX or BOXEE (Is that still a thing?)