Dear Louis
Thanks for sending the direct message to call for my attention on this particular job/project.
Thanks for comments posted by you all regarding my projects.
I agree with Wallace that the best way to go here on this w111 280SE-3.5 and the other 250Se where you plan to install power windows is to use the system that came on w108 which uses power window motors with only two contact leads, because these motors use more modern power window switches which can be sourced new everywhere. Older three lead motors, even when are more robust, have the problem that they relay on early power window switches which are terrible expensive, and most of the times as used parts tend to fail. Early power window switches have three leads. The central cable is ground, which is switched either to the upper output cable or the lower output cable. Early switches to do not reverse polarity to change power window motor movement clockwise/counterclockwise., they only send a negative signal through one output cable or the other. The power window motors have a separated winding for the movement UP and another for the movement down. The early motors have constant 12V on the central black cable and receive the ground for each of the separated winding’s on the green leads.
Two lead motors have attached a gearbox which is much better that the approach used on early w111 which used a motor coupled to a conversion box via a plastic/rubber sleeve link. That part can be sourced with Wallace new in case you have the three lead early power windows motors.
On the modern two lead motors the polarity inversion, to make your power windows roll up and down, is made by the power windows switch itself.
I think that you can source a w108 power windows harness and install it directly with minimum modifications on your w111. Also the power windows motors could be sourced from a later w108, years 1970 onwards. They fit on your w111 with minor modifications to none. Just be aware that these motor are sold as a set since for each door they have differences. So you should buy one FRONT LEFT, one FRONT RIGHT, one REAR LEFT, and finally one REAR RIGHT motors.
If you need one power windows harness already refurbished, I have some here at home.
Best regards
L.Peterssen