The undercarriage car washes are fine, what do you think when you are driving on the highway in the rain? I find the touchless washes put a little too much wax on the car. I tend to go to the self serve and use the spraygun. Once a week, 20 weeks on winter (depending on where you live) $5-8 a wash = $100 to $160.
You MB has corrosion protection on it, most metal is galvanized plus any painted metal has I think at least 3-4 layers of primer, rust protection, paint, and sealent. The only piece of metal that is more subseptible to rust is your exhaust at the welding joints. You disc brakes are also exposed, but when you use them, any surface rust is immediately grinded off.
If I can remember from my high school chemistry class, in general salt water starts to corrode exposed (non galvanized, non treated) metal (steel, iron) a few degrees above freezing. If you have your MB in a heated garage, you should have the undercarrige washed more often. If its left outside or in a unheated garage, the chemical reactions should not occur especially if the temperature is below freezing (0 celsius).
Please note if there is no water there will be little or no corrosive reaction. So if you car is dry with some salt residue on it rust should not occur.
I haven't had any rusting exhausts or metal since the early 80s. The new cars have top notch rust protection in the metal to prevent oxidation and rust.
Ask yourself, when was the last time you saw a rusty MB? :?:
Why would you want to drive your MB if its dirty and full of salt?