1) the weight of oil you use will affect the oil pressure when warm, I would expect that the oil pressure at idle, in gear, and warm after driving 75 miles would be 1-2bar or 15-30psi range with 20w50 oil in the engine.
2) The rough idle problem could be weak motor mounts(causing you to feel more of the engine vibrations), Loose A/C belt vibrating (have heard them where it felt like the car is comming apart), or possibly an idle rack limiter that the spring is weak which allows the injection pump rack to oscillate too far causing a "rocking" affect in the whole car.
That car was made before I was concieved, and I have only had the opportunity to work on one (brakes). Sorry can't help.
14 year MB tech
7 year shop foreman @ independent MB/Volvo shop