RoonServer is a Core/Output lacking only the Control from a full version of Roon. So yes, a NUC running ROCK will expose it’s USB outputs to any Control under the “Connected to Core” section in Settings/Audio.
Other non-NUC Core devices exist. Roon comes in Windows (32 or 64) and MacOS versions. RoonServer adds x86(64 bit) Linux. Anything that can run those OS and offering i3 or better performance would be good (i5 for a larger library). I happily used a Gigabyte BRIX (now running ROCK). MacMinis or repurposed laptops can be used.
These are examples of ITX builds for $400 to $800 that will run a Core. You don’t need the video card either, which saves a further $100 or so. I spent a bit more to kit out the CoolerMaster ITX case which is my current Core. These all definitely count as building your own PC though.
The SonicTransporter i5 is a purpose built Roon Core for $645 that will cope with libraries up to 450,000 tracks.