Personally, you be a much stronger person doing this if this is what your dream in life is. For me though, I am not even considering medical school, and if I was I would probably get a student loan (or something) if I was. If I had the calling for the military, or if the United States drafted me then I will go with honor.
It's your decision, and that's basically a quarter of your life, give or take. You probably could also become a doctor in less time, then in 19 years, probably more like 10 years or less after becoming a doctor and paying it off.
If this is what you really want to do, and you feel like you need to do it. Then do it. Or otherwise you know what the result is.