Originally a Dell Optiplex 980 USFF that I used as a HTPC, I eventually decided to cobble together a NAS from my parts boneyard. I transplanted the CPU and RAM from the Opiplex along with 3 1TB HDDs, Intel SSD, and PSU into an ancient case I had gathering dust. The motherboard was just the cheapest board I could find on ebay with the right amount of SATA ports and socket, plus it came with an 60gb "Samsweet" mSATA drive that I figured I might as well use. Being a full ATX board though does mean that I can't actually put the case cover on anymore, since the board hangs out the side but that really doesn't matter. The heatsink was bought new since the heatsink from the Optiplex couldn't be re-used. The two drive cages came from Amazon since the stock case only had two drive bays, they've both been great though the EverCool one will probably get replaced with an IcyDock hot-swappable version in the future. The two 3TB Seagate Constellation drives were added a few months after the initial build when I ran out of space on the original boneyard 1TB drives. The Kingston SSD replaced the no-name mSATA drive when that died, which was being used with the Intel SSD as a cache pool. The GTX 970 came out of my gaming rig when it was replaced with a 1080 and gets passed through to a Windows VM.
Overall the system has worked just as intended. Unraid has been a breeze to work with, especially for someone with virtually no Linux/Docker experience. It's primary function is as a Plex server/NAS and occasionally streaming PC pushing the feed from my gaming rig to OBS on the Windows VM via NDI and utilizing the 970 for the transcode.
In the future I expect most of this to merge with my current gaming rig once I refresh that machine so it'll actually get a new (to it) case, CPU, and RAM at that point. I'll probably also just replace the remaining two 1TB drives with another pair of 3TB Constellations when I need more space.