hi mate, firstly welcome to LOC, and looking at your display pic, nice car. cant help too much with the parking sensors. but if you really want to fit the parking sensors yourself, why dont you convince your other half, that she owns the other half, and you can do what you want to the back? also, where are the scratches on the car, if they are on the bumper, ask a bodyshop to fit the sensors and repair the scratches, also, have you tried polishing the scratches out, as they dont sound too bad from you`re description.
Anuj