If you plan to keep the vehicle for much longer, consider this: purchase a new evaporator box (124 830 80 60?) from M-B.
It will be the modified, new style that holds the intake air filters for the blower motor. And it comes assembled, equipped with EVERYTHING, except the air temperature sensors and air filters: blower motor, blower motor regulator, all vacuum elements and hoses, new style evaporator, expansion valve and heater core are all included.
Find a dealer who will sell it to you at wholesale (some dealers do offer price reductions for those suffering from leaking evaporators).
Crunch the numbers: on a older vehicle worth keeping, using a new box can be cheaper than replacing parts piecemeal. Plus you get the benefit of the cabin air filters.