OP wrote above:
[[ If the 2014 minis aren't user upgradeable is it workable to run OSX off an external SSD and use the internal 1TB hardrive for storage only? ]]
Yes, of course it is. I've been doing that for almost 2 years running now on my late 2012 Mini.
[[ I presume you'll lose some performance going through USB or thunderbolt.
How much? Is this a viable option to get a cheaper SSD involved? ]]
If you "lose" any "performance", what is "lost" will be all-but imperceptible to you.
I suggest building up a "USB3 external booter" with the SSD of your choice. You can put it into either an external enclosure, or, you might use a USB3/SATA dock instead (which is what I do). No difference between the two.
BE AWARE that there are a couple of limitations of using USB3:
1. You won't be able to enable TRIM, and
2. You probably won't be able to run firmware updaters.
HOWEVER....
.... as I mentioned above, after almost two years of booting and running my 2012 Mini from an SSD mounted in a USB3/SATA dock, neither of the two "issues" above has proved to be "an issue" at all. "TRIM" seems to be becoming less of an issue as SSD's evolve.
If you go the thunderbolt route, the issues above won't come into play. But you're going to PAY considerably more for this privilege. The choice is up to you.
