Roon wouldn’t be Roon without it’s metadata. 
It’s up to you to choose the right hardware – depending on your needs – to run Roon. For my personal needs – no streaming services, no excessive use of DSP – Roon runs beautifully on my QNAP TBS-453A-8G (so all SSD no HDD) with a Celeron N3150. But was it cheap?
Buy yourself the cheapest AIO PC with SSD, touch screen and FHD resolution you can find. Most likely it will contain a better CPU than a Celeron, so just run Roon Core on the AIO instead of the NAS and keep Roon’s metadata.