Fitting a stiffer rear bar will promote oversteer.

A mercedes is designed to be very very neutral with a touch of understeer because understeer is safer than oversteer on the highway (or race track).

To improve handling I recommend:
1. make sure the suspension is in good condition.
2. Add hd bilsteins or Koni shocks.
3. Excellent tires

For most folks I would recommend stopping at this point.

If you still have a bug you can cut some spring off. this is inexpensive and will lower the car and make the springs stiffer.

Heavier sway bars front and rear would be the next most cost effective solution in most cars but none are avalable for the 123 chassis so your best solution is having bars bent or swapping in wagon bars front and rear.

Lowering springs are another way to help some but are virtually unobtainable.

For the cost of modifying your 123 to really handle significantly better you can by a ten year old Miata and drive that when you want to carve up corners. Come to think of it I have a 99 Miata I will sell you or my old 280e euro autocrosser which had h and r lowering springs on it and hd bilsteins in front and Koni adjustables in back.

Really for street driving it is hard to beat a stock benz with all the components up to snuff with stock sized tires.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.