you cant add a motor to the existing one unless its a cheep ebay kit. but how u fit a linkage over a winder the wrong side of the door card im not sure!
early mk2 golfs with quarterlights have similar shape lifters which can be made to fit.
vw Kit was made by electric life they are still availabe but are nearer £300 ( but the switches are obsolte so u get substitute ones).
Mk2 golf 4 door rear ones are close ( upright bolts straight in) but u need to make a step down plate ( bit of 3mm metal with 4 bolt holes) as the motor sits lower than the polo version. (i have a set of these in the house if your interested).