The new Mini’s are hard to upgrade, so get one with the max amount of memory you will possibly use over the next 5 or 7 years (roughly the feature-lifespan of a computer these days). Second, it makes more sense to get a relatively small internal SSD for just the software (OS and whatever audio applications you would run). That drive will be fast (SSD) and you won’t really need to back it up as frequently as you would back up a growing media library.
Keep the media library on an external RAID device to minimize the need for backups every time you change the library, but nonetheless figure out a reasonable backup of the RAID, a fat drive for that purpose can be very inexpensive. I always keep at LEAST three copies: The original operational library disc, and two backups that I rotate between. I have had disc failures, I know of many people who have had disc failures — it just makes sense to have this insurance. (One issue with a RAID stack is noise, so factor that into where you physically locate the music hw stack.)
If you can afford the price differential, go with Thunderbolt to RAID, it’s more but performance benefits. Do some research on the best digital interconnect between your Mini and your DAC... I am using optical to my DAC, but it’s really not the best. My Mini also serves up high resolution files to a Cambridge Audio Azur 851N — via Ethernet, on a separate segmented network in my home (again for performance benefits due to no network congestion).
Configure the Mac OS to cut-out applications or startup items that you do not use in headless server mode, and those CPU cycles and memory won’t compete with your music applications. You can search for terms like "minimal processes for Mac OS" or "minimal set" or "only necessary processes for Mac OS". Take the same approach with cases you plug in, a dedicated music server should have few.
As far as the model to buy, I would aim at the latest model/chipset because I would not want to do the same thing again within say 5 or so years... It really should serve well for at least that long. ...and get a UPS for the Mini and RAID, I consider UPS basic insurance for computer electronics (analogous to power regenerators for audio gear).
I like the idea of specialized music servers, but I prefer to roll my own using a Mini because I am quite happy with the open source audio software and do not like vendor lock-in for dedicated commercial music servers. My mini does everything I really need, and when I don't like some aspect: I can change it by swapping server software or adding other software. This means I have far more flexibility at a time when these devices quickly become eclipsed by other devices -or- when I can't tolerate some user interface nits all software seems to have...
