I'm not a big fan of Apple either, but do use it (primarily because employers are more willing to support/allow Apple than Linux and I just can't do windows, so lesser of two evils really).
However, I have upgraded my old mac mini to High Sierra with no problems. I can't remember when I actually purchased it, but from vague memory, it originally came with mountain lion, so that would be 10.8
I believe 10.12/10.13 are supposed to be supported on all mac mini, imac and macbooks from 2009 models onwards. My macmini is slow, but it was always slow. I do run Emacs on it.
Tim
P.S. I also will be putting Debian on my Mini at the end of this year.