Look at the SonicTransporter from Small Green Computer. It is less expensive than a Nucleis and offers more cores in the processor (in the i9 version). Some will say that Roon does not use the extra cores but you can get HQPlayer built in that will use the extra horsepower. I upgraded from a NUC i7 to a SonicTransporter i7 about 3 years ago. The SonicTransporter is much faster than the NUC for searches in addition to library processes. Small Green Computer also offers outstanding customer service.
Supermicro is used a lot in business servers. You can find them in Newegg Business, CDW, maybe Amazon.
I start with motherboards designed for WorkStation use and go from there. Those are not usually small form factor though, and that is fine by me. My last build was based on the Designare Gigabyte board.