It's mystifying how decent hardware gets chucked out. I recently started investigating Kodi for a media server, looking around for cheap hardware I sourced an Dell i3 with 8gb ram and an SSD for £45 including delivery! I installed Manjaro Linux ( for no other reason than to learn about it) and Kodi and there you go, a music server.
The only observation I would make, is that with very old hardware, the modern websites will not load properly or only very slowly. My guideline is it needs to be an. i3 and be able to run a 64bit distribution.