Replacing a MAC Mini with new one...

Mesch,
Recommend getting the max amount of memory. If you later decide to try a different software package such as HQ Player, you will be glad to have as much memory as possible. Also, as file sizes and your library grow, loading art etc. will be faster with more memory. 

Not to derail your original thought process but have you also considered alternatives to the MacMini approach including keeping your library on a NAS in another room and going with a low powered streamer/renderer to play the music on the NAS (and online if interested too)? There are many products to consider and my experience using MacMini for years is leading me down that path. Computeraudiophile site has the detailed threads on most of the pros/cons to this approach and specific recommendations.
Have fun either way you decide, Cheers,
Spencer 

Mesh, your USB/SPdif converter strips timing from the data.  In asynchronous USB data is delivered in packets at its own timing that has no connection with clock used by D/A converter.  Furthermore, Pure Power prevents jitter induced by noisy computer supply.  Pure Music is fine, since you already have it.  8GB is not necessary now (I run El Capitan on notebook with 4GB), but is probably a wise choice for the future.  Yes, you can use the same USB, since at this point only data (and not the music) is delivered to your converter (slight delays are not important).  Situation is different on SPdif side where timing is crucial.  I believe that your converter has both Toslink and coaxial outputs.  Normally coaxial should be better but try both if you can.  It is system thing and in some cases Toslink works better.  If you use coax, try to buy or build one, that is very short, less than 12". The shorter the better.  If you cannot do it then get 1.5-2m.  In first instance you won't have reflections in the cable (from the end of the cable) affecting transition while longer >1.5m cable will delay reflections making them miss original transition.  Try different digital cables.  It is characteristic impedance matching and more expensive cable is not necessarily better.

Get the least expensive slowest new MAC.  SSD is not required but it is quiet. Use power filter for your DAC if you cannot use different outlet (different phase) for computer. 

Sbank, I have and continue to consider other options. Would consider a basic server that could read AIFF files off a HD. 

Kijanki, thanks much for your input. I am using BCN connection between converter and DAC, direct connection w/o cable. I have 3 dedicated lines, one for digital.

In my household I have a Mac book pro which I use for general computer use and to serve music to my secondary systems, and the mini for my primary system.  What are thoughts in usng the MAC pro to burn CDs  and transfer file to HD and then using the HD with the Mini?  

Mesch, that's same as my setup. I started using the macbook instead of the Mini to rip and download files that I store on the NAS. My plan is to sell the Mini when I receive my Sonore microRendu which will be available soon. Aside from concerns that the slide out drawer on the macbook is a little flimsy, no problems. As more of my new files arrive via download vs. disc rip, this will be even less of an issue.
The only real quirk I've had to deal with going to NAS from HDD connected to the Mini is that if I let my Macbook "go to sleep" overnight rather than shutting it down, sometimes Finder's ability to see the NAS gets lost overnight as I have the NAS automatically hibernating during the night. Rebooting the Mac in the a.m. fixes it. Cheers,
Spencer

Thanks for the confirmation Spencer! 

As I have time prior to making a move, I plan on continuing my research on this, and looking into the possibility of a server.  There is a Bryston BDP-1 for sale here on AG. May look into that as an alternative if it will play AIFF files from a HD. Would compliment my BDA-1.