As far as sitting in front of a computer all day, that is definitely a possibility with any engineering job. Generally though, if you make your interest to get your hands dirty known to potential employers the recruiters will either not offer you an interview or guide you towards groups or positions where this is a possibility. If you don't want to sit in front of a computer all day ask questions of companies that interview you and make employment decisions based on their answers.
This is general, as I'm unsure of what the job duties of all mechanical engineers at BMW are, but if you are interested you should contact them for information on their co-op and/or intern programs. That's the best way to find out about a company and get an offer after you graduate.