If you want to keep costs down it may be better to go with one without two drives , but just one drive, unless you have buckets of music files and videos. I use one nas drive and then mirroring software on my pc which mirrors a music drive on my pc to the nas. A lot of people don’t use anywhere near the storage of PCs, so it’s a good place to start storing music. If your pc etc is full or nearly so then buying a two drive nas makes sense, as you can backup one drive to another on the nas, but I’d maybe explore the possibility of solid state drive nas’ to keep noise down. I’ve really like the Melco and innuous drives which are expensive but eventually I’ll get one when I can I think, as I think all the functions they give is bound to have an effect on sq, albeit probably not massive. I know with synology you buy the enclosure and the drives as Mark says, but you want a drive with best possible processor speed for the price, highest storage, and check out if you like music server structure etc.
I use a programme called ‘anywhere back up’ to mirror my drives. Another really important feature of nas drives is whether they have software that can purge deleted files. So if you decide to mirror files between pc and nas, you want the file on the nas deleted when you delete the file on the pc. Some of them fill up and you have to do this purging manually which can be a faff. Eg when you buy one track from the album online in low quality, then buy the cd album, and re rip the album, you want to delete the single file and add the album.