I don't think you could ever get a 124 e-class with pass-side memory seat. BUT...
you will need the switch with extra buttons, the wiring harness which connects it (a duplicate of what's in driver's door) plus all of the hardware from the driver's seat. Plug it all in, voila, pass-side memory seat in a 124. As you move the seat, the motor sends impulses to a small memory module for every revolution it makes. The module "counts" how many impulses the motor sends as you move the seat away from the initially "memorized" setting. Then, when you press the memory button, the module supplies power to the motor for the number of impulses each seat motor moved away. That's how it works! I have memory for both seats in my Range Rover, but never use the pass side.

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2002 Mercedes-Benz E320 4Matic
1995 Mercedes-Benz E320 Wagon
2005 Land Rover LR3 V8 SE
1999 Audi A8 4.2L quattro