I've got a basic question: Why don't you get some good snow tires and drive it instead of garage it? The worst thing you can do for an MB is to let it sit idle. They do best when driven regularly.
I drive mine 365 days a year, even in the snow and salt. So far, there is no apparent damage to the undercarriage from the salt. I just get it washed once a week and don't worry about it.
These cars are meant to be driven, not be garage queens.