Hi,
It is possible to retro fit anything, its just if you have enough money and time to do it.
You would need to identify what parts you will need I would be asking myself questions such as,
Has your car already got electric seats? If so then you would need to replace the entire wiring to the seats as memory seats are fed from control units which are part of the cars CAN system, and non-memory cars are fed from a simple relay and multiple feeds from the rear fuse box, you would possibly have to replace the seat motors as well as they are different.
Have you got an electric steering column? This would have to installed and wired up to have the fully functioning memory system.
The memory system also stores different memory programmes specific to which key is being used so there is probably another control unit somewhere to do this.
To be honest if i was to attempt this I think I would want a donor car and remove the entire system from the car carefully documenting where and how the looms are run, control units mounted and to ensure I have all the parts, then it would involve some clever STAR work to activate the system.
I,ve retro fitted a full electric, heated, Orthopedic leather C43 interior into my C230k which just had non heated non electric cloth interior from the factory which was quite straight forward, but I would not of attempted it if i had,nt had a donor car to remove the wiring from.
If you are looking for possible retro fits for your 203 Peter (whitenemesis) is your man, he has completed many on his car.
It would also pay you to get conversant with the WIS and EPC so you can study what is required in each of these fits.
Good luck