Vibration under braking is most likely warped discs (you call them rotors on your side of the pond) but have the wheel balance checked first, front and rear.

Groaning from the front end as the car comes to a stop seems to quite common, if it at speeds below walking pace it is not the ABS as some people think as this cuts out below 5 mph. Both my 124s have done this, it may be worth replacing the front pads for original MB ones.

The straight line stability thing is not so easy to discuss. You seem to have checked everything although you have not said anything about tyres. Over or under inflation could cause this as well as uneven wear resulting from previous misalignment. Don't forget that these cars have old fashioned recirculating ball steering, unlike a modern rack-and-pinion system there should be some play in the steering and these means that minor course corrections as you drive along require larger movements of the wheel.

__________________
Mick J
'08 Chrysler 300CRD (MB OM 642 engine)
'95 E220 estate
'89 230TE (R.I.P.)