JFYI, Roon on Windows is going to outperform Roon Mac or Linux box, assuming the hardware is identical. Roon is mostly build on the .NET framework, and we’ve seen that Microsoft’s implementation performs about 20% better than the Mono solution. This may not be the case in the future, especially with Microsoft opening up their runtime, but it is the case right now.
You can probably take the fastest fanless cpu you can find, run Windows + headless Roon on it, and have a pretty ultimate Roon box. I like the various fanless Core i5 NUC boxes 
Combine that with the various hardware manufacturers releasing RoonSpeakers capable devices, and you have something that will have fully Roon, silent, and with audio quality that can’t be beat for your price range (no matter what it is).
Headless builds for Windows and Mac are going to alpha testing next week, released soon after. July is going to be a fun month for new Roon configurations.