What's the difficulty of minoring in Spanish if you're a non-native speaker?

College language classes are designed for non-fluent speakers, even the very hardest ones. I would be much more concerned about your required pre-med classes dragging down your GPA than a handful of Spanish classes for a minor. If you've generally done fairly well in Spanish in high school (As or Bs) you should have no issues getting decent grades in college-level Spanish classes.