I got a '97 S320 6 months ago. 109K, Dealer serviced.

Lower control arm bushings worn, had them replaced. Visual inspection should give a clue. Age is a factor. The rest of my suspension was pronounced fine by my MB indy.

Thing that jumps out in your problem description is "cut rotors" and vibration on deceleration. That's typically a dead give away to rotor warp. Consensus is to never turn, cut, MB rotors. New ones are less than $100 bucks each. Factory is best, Balo, Zimmerman next. The prior owner, local guy, knew he was going to trade up and had the rotors turned at a gas station. Even though they worked fine, I replaced them right away with new Textar Yellow box pads.

My vibration was slight, came and went, between 70 and 80 mph. New Bridgestone Turanza's balanced at the tire shop that sold that brand in my town. They used lead adhesive weights. I took the car to my normal tire shop, with the Road Force machine.
We agreed to use the plastic coated rim weights. As was the case on my 300E's, vibration problem solved.

I have also recently replaced the front shocks, with Bilstein HD's. Noticeable improvement in firmness and compliance. The OE shocks were Bilstein. They probably had another 100K in them, but I am anal.

Don't know that it relates, but which W140 is it? S320, S420, S500? Also LWbase or SWbase?

Steve