I don't know how they do it in Munich, but normally u will have a supervisor u is an assitant or a Ph.D. Student of the professor that "supervises" the thesis. He will supervise the thesis but he won't be your main contact, normally u will talk with your "small" supervisor and u will have a few meetings with the prof u will then, yeah talk with you about your thesis.
And working in his group, or writing a thesis with his group shuldn't be a problem, just make sure u do well in the lectures that are close to his subject and u will heva a good shot. (Could be that there are more people who want to write their master thesis's with him so I think that their will be a cut off, but if u aim at a good Ph.D. programme u will need good grades, too)
Good luck
