Become a Full Member and get the official Linkage Tour and the official Starting Aid. This information alone will pay for your membership
I'm not trying to ruin anyones business, but I just can't understand why you would replace one nasty aftermarket unit with another one. For an original w111 Behr ac unit expect to pay 2-3000 euros, and that is just for the under dash unit. And that's if you are lucky enough to find one.
http://www.mercedesheritage.com/classifieds/viewlisting.php?view=1526