Do you have any worn suspension parts identified? My 300E has almost exactly the same mileage as yours and everything is still very tight.
I put in the Sportline sway bars with great success. The mistake I made was using KYB performance shocks. They are way too stiff. I would recommend Bilstein Heavy Duty or KYB standard. The Bilstein sport are for lowered cars. If you decide to use the sportline springs you will use the Bilstein sport.
By staying with the standard springs with slightly stiffer shocks and using the sportline sway bars you will maintain the resiliency of the suspension, but gain the sway control of the sportline suspension, this becomes a handling advantage over the sportline when you are on rough roads.
Replacing the rear sway bar is not too bad if you have a good controllable floor jack, a set of good jack stands and are VERY careful. The trick is to lower the rear suspension subassembly just enough to snake the sway bar in and out, without lowering it so much that you strain the brake hoses.
The other thing to do would be to go to 205/60 15 tires when it comes time to replace the tires. You can also go to 16'' wheels and go to a 50 or 55 series tire. These low profile tires will help a lot but stiffen the ride. If you stay with the stock springs and stiffer shocks, this is a good combination with the ultra low profile tires.