It is not as bad as it sounds depending on how far you want to go. The general public will not be able to guess the model year of your car as long as it is in like new condition.
The new rear shelf is noticable to a Benz person because of the rear center brake light.
The tail lights and rear bumper cover are not that bad of a swap but go a long way to updating the look.
The side skirts help get rid of the lower black areas along the bottom of the car that are a dead give away that the car is the pre-face lift model.
The most difficult part is the front bumper but most people don't see the front end for long in their rear view mirror. So I didn't change mine.
I would recommend changing the side moldings in the doors and front fenders so that the line of chrome matches. I have not seen anyone change the door handles and you don't need to worry about the inside cluster.
Change the mirrors to blinker mirrors for the most functional part of the conversion.
My E-Class is a 96 and I get compliments even at the dealership.