I recently installed new Bilstein shocks front and rear W126 '91 300SE.
I ordered the standard shock, NOT heavy duty. There were no boots or plastic covers for the rear, just for the front shocks which I reused the original that came with the car. My mileage was 120K. I had no problems.
The rear seat removal was the most difficult part of the whole deal. I do not think the rear shocks will get that much water spray or whatever as the front ones will.