The 15-20 hour flat rate and reported times for evap removal and replacement were for the 123.... generally 1980 -1984....
So maybe they had corrected this situation by 1987.... don't know...
Usually the evaporator does not need servicing from a leaking perspective... but it might be nice given the age if you can look and clean the upstream fins.... since air does have to transverse that area in order to cool you.
If your car has the same general system.... block valve next to the evaporator.... then you may want to install a new one... about $35 on our cars.... just on principle...
You did not say why yours is not working... or what you have done to it.... and I do not have a manual for your car ....
Do YOU have a MB factory service manual for your AC system to look at ? More specifically to read any warnings or instructions concerning flushing , etc ?
If not... I do not give you a 50 percent chance of getting it going properly THIS weekend.... Weekends are much harder for acquiring parts down here... so any two days not a weekend ups the chances for success within the time frame considered...

What AC working tools do you own or have available ?