The W124 series didn't get a cabin AC filter until 1994. Maybe the 500E got it earlier. It's a lot of work but only bolt on work to adapt a 94-95 cowl and ACC airbox to retrofit a filter. An outfit in Japan sells filters that fit over the cabin air intake grates astride the wiper pivot in early W124s. A filter set was $80 plus shipping 5 years ago.

I think your motor's intermittent operation is caused by worn brushes or bearings. I'd gamble that the blower controller is in good shape.

Sixto
87 300D