Information is in this thread: Simplified Roon configuration: no network or NAS.
I bought a standard NUC and memory and SSD storage at Amazon. Hardware was easy to assemble. Installed Windows 10, then did some tinkering for the automation.
The NUC has a fan. There are fanless equivalents available at Amazon and elsewhere (@Andybob uses a BRIC). At the time the fanless machines didn’t have Intel’s latest chip (5i5), probably do today. The fan is not audible, but I want to get rid of it so I will transplant the innards into a fanless case. Should be a fun afternoon project.
The 2.5 TB SSD was over half the cost. You can get less if you don’t need that much; I currently use less than 1 with about 1,300 albums, some high res, most CD.