Wash it often, ideally with fresh water, as recirculated automatic car wash water in winter has more brine than the ocean. You may want to have the undercarriage steam cleaned and explore the possibility of an application of aftermarket rustproofing, although many will claim that it is unnecessary or even counter-productive. Although commercial self-wash systems also recirculate some water, I still use them in winter to flush out the wheel wells, especially along the inside of the flange located around the wheel openings. Make an effort to direct water anywhere the body design traps ice, dirt, etc.. Keep the body drain holes open.

You are right about a garage accelerating rust formation. I found this one out the hard way.

Starting with the W126, Mercedes became serious about the use of materials and design techniques to combat rust formation. I live in NE Pennsylvania, where terrain and temperatures mandate road salt use. I can tell you that I've never encountered a W124 of any age with overt rust damage. On the other hand, I remember a lot of 5-year-old W108's with rust bubbles around the rear wheel openings.