jwp wrote above:
[[ Running off of the USB would be too slow. ]]
This is nonsense, pure nonsense.
There is nothing "slow" about running a Mini via an SSD mounted in an external USB3 enclosure or USB3/SATA docking station. Granted, 430mbps read speeds do not match the 730mbps (or so) speed of a "blade" type SSD mounted internally. But 430mbps is a world apart from the speeds you'll see of a 5400rpm HDD internal drive.
Attached below are the speeds I get on mine (Crucial m500 mounted in a plugable.com USB3/SATA docking station. System has been up and running this way since March 2014):
Also -- from others' postings here, it looks downright easy to add a "blade" SSD to a 2014 Mini, IF you have both the blade AND the proper ribbon cable/mounting platform parts. The ribbon can be "detached" from the mounting platform and re-located into the Mini with minimal fuss and bother...