Ask HN: How to DIY build a NAS system with old PCs and open source software

I have searched the HN, nothing new since a few years ago on this. So here we go: I have a 2 year old low end desktop PC laying around with Celeron processors, rather than throw it to the landfill, I want to build a NAS system (pretty sure it has horse power than the custom ARM chips in the commercial solutions out of the shelf). Gigabit NIC upgrade, cheap hard drives and RAID setup (RAID5 or something else?) are assumed. I wonder if anyone in HN community have had similar DIY project built and with good experience with them. Obviously, it would be run on an open source software stack (but which ones should I choose?). Any suggestion is appreciated!