Also agree that your plan is fine and no real need to have an SSD external hard drive. I use a Western Digital My Passport Studio, mainly because it has a firewire connection, but matching the cosmetics of the mac mini is nice. Both are two feet from my ear and what little noise pops up very occasionally is barely noticeable.
I'd recommend using USB over the optical, which doesn't sound great. If your DAC doesn't have USB, I'd suggest getting a USB to SPDIF convertor. I use a Bel Canto Mlink, but even the Music Fidelity VLINK II at lesser cost should be more than adequate.
And has been said elsewhere, if using USB for the DAC, use something else for the storage - i.e. firewire (or Thunderbolt). This way they are not sharing resources. Since using firewire for the storage I've been stutter and hassle free - something I couldn't say when I had many things hanging off the back of USB.
Alternatively if your music collection is not very large, just go for the 1TB option vs the 500GB on the Mac Mini and save yourself an extra box.