3 months beforehand is more than enough.
While I didn't use KMK, I've heard mixed things about its usefulness from classmates. Some lived-and-died by it or enjoyed its organization, but some thought it was not worth the money and could've passed without it. I'd say it was about 50% satisfied and 50% unsatisfied amongst the sample I asked.
Tips: The more often a topic comes up in the curriculum, the more likely it will be on NBEO - your school wants you to succeed on the test so they teach core topics with repetition. ex: myasthenia gravis was discussed in some form or another in 5+ different classes (clinical diagnostics course, pharmacology, neuro, ocular anatomy, human and ocular physiology, oculomotor, etc courses all touched on it briefly) and lo and behold myasthenia gravis was on NBEO. Tetralogy of Fallot was discussed in 2 classes so its likelihood to appear is not as high but still something that can occur.