Here's my two cents worth.
1) The amount your 300E will be lowered will vary. It has a lot to do with how much shorter the springs you buy are and the thickness of the spring pad used. I have an AMG lowering kit on my 300E. The kit lowered my car approx. 1 1/2 inches front and rear. I still have stock size wheels and tires.
2) A good set of Bilstein sport shocks MAY stiffen the suspension enough so that the suspension travel will be significantly less therefore preventing tire rubbing.
3) Depending on the clearance you have between the fender and the tire will dictate if this would work. Given the fact that you do have 18" rims tire rubbing is a high probability.
BTW, for what its worth MB shocks and struts can and do last well over 100K miles.
92 Volvo 740 Wagon aka "Mutt mover"