Well, I don't know about raining on your parade, nor do I know very much about the actual expenses involved, but here's some other information that might help.I've got a friend who teaches high school English at a school in California and she has studied Japanese extensively on her own. She can speak Japanese fairly well (with an accent, obviously) and get her point across, but her vocabulary is still limited so she doesn't always understand what is being said, and her listening skills are fairly poor. We met in Japan, and by the time we both left, at the end of our year of teaching there, my listening skills were almost on par with hers, and I went to Japan with the ability to count to 10 and say "Good afternoon." Bottom line; if you want to be fluent in Japanese, you should live there.
I teach at a small technological university in the Midwest of the U.S., and we have students of various nationalities that come here to study. The vast majority of those students have to spend a minimum of one year doing remediation of their English, to get them to a point that they can learn in an English speaking environment. They are doing engineering work, which is mostly in the mathematics anyway, and they still have to spend a good chunk of time intensively working on their English, despite the fact that many of them have studied it sense they were in grade school. For you to learn Japanese and then use it in a highly specialized setting like medical school in Japan seems a bit ... ambitious. From my experience, the field of medicine in Japan has its very own jargon, which you would need to learn in addition to normal Japanese, and the English medical jargon. I'm not saying that can't all be done, but if you want to climb a mountain, you might not want to start with Everest.
Might help for you to think about this; what are your goals for being an MD in Japan? What distinctive thing do you hope to accomplish there? Is it possible for you to accomplish it through a slighly less direct route? Maybe earning your MD in your home country while studying Japanese would be a good idea; perhaps you can eventually migrate to Japan as a doctor who is fluent in Japanese?
As I said, I don't have a lot of direct insight into this, so take that for what it's worth. Best of luck to you!