Over the years I have done many heater cores but never on a 123 though. If you have reasonable mechanical skills and some patients you can do it! Just think of it in stages or a series of small jobs rather than 1 large job.
Center console has to be removed. Thats no big thing
Steering wheel
stereo and climate controller
glove box
Before you know it it's out. Just remember Germans over engineer their cars so a small handfull of screws left over should be acceptable