Knowing Japanese is a big plus for getting a Job in Japan, and if there is a high demand for your skills, that's even better--- Actually, being self employed hurts you rather than helps you unless you are insanely rich I suppose--- a valid working visa will help you to stay in Japan longer than a few thousand dollars comming in each month.
The best bet is to do a job search for your field and see what is available, and under what situations they are hiring.