Nucleus does not support UPnP, therefore you won’t be able to use Lumin app to access the library on the Nucleus. This is not a problem in case you’ve decided to only use Roon to play music instead of Lumin app.
If you simply want a fanless Core, then buy a NUC10i7 or NUC8i7 with a fanless chassis with no less than 16GB RAM. Install Windows, Roon, and MinimServer (UPnP server).
A nucleus does open external storage as a shared drive. You can set this up as a drive letter in a Windows PC (as I have done) so you can just then copy/paste new music to it.
Although, I do want to second Peter’s advice about a new core. But, what is it that you are hoping to achieve by changing to a new core?
My connection doesn’t seem very stable (dropouts), and I’m hoping the Nucleus might help this. I have to use powerline ethernet and I’m not sure this is as stable as I’d like, so I may be wasting money on a Nucleus if I need a direct-wired connection for best stability.
I’ve also read that the Nucleus might sound a bit better than the PC Core.
A nucleus will not help as it is just a PC in a different case. There is no SQ difference.
Your network issues probably stem from the powerline ethernet. Try running an ethernet cable as a test and see if stability improves. Even if you can’t leave the wire there, you will at least know for sure that is what is causing the issue.