Have a read of this article. It will probably answer most of your questions:
A NAS is an expensive way to set up a Roon core, though the right hardware can offer certain advantages over a NUC/Nucleus.
You want the database on a SSD. Music files can be on HDDs.
I wrote a response to someone having issues with a NAS, expanding on the pros and cons of using a NAS to host a Roon core. It may give some additional insight:
BTW, I’ve subsequently upgraded to DSM 7 with no real issues.