Upgrading from a Mac mini (2009)
Get the non-server version of the Mini. Put a Fusion drive in it, and fill it with RAM! You will LOVE this machine!
Now. Get an external Thunderbolt drive for your library! It is just as fast as an internal drive. If you get SSD externals, it is expensive but fast! I use a Drobo 5D for my external storage. I connect it with Thunderbolt and I am VERY happy with the speed. You can also put a SSD in the bottom compartment of the Drobo 5D to really speed it up if you need too.
So, Get the Mini with Fusion Drive, add Ram, get Drobo 5D. That is what I suggest....