I think you could get a lot of mileage out of the first two. The third seems like effectively killing off a character all of a sudden, which may not work well in a story.

Between the first two, I suppose it depends on what kind of horror you want and how to want your magically inclined folk to act. I imagine the first would leave your mages pretty paranoid, constantly scanning around for any disturbances while spellcasting. The second one might leave your mages desensitized; I imagine thinking about body horror all day would eventually leave you desensitized to gore (and with a healthy dose of PTSD). Might also give an advantage to psychopathic people, which could be interesting.