I've got the same car and similar mileage, also passed down from the family!
You don't mention how long you've actually been the driver of the car so maybe this is a moot point. But if you are new to the car let me tell you something about it's turbocharger.
I've owned three cars with turbo's and the 300D's behaves unlike the other two. First of all, a whine(whistle) is not unusual for a turbo, if the sound is loud, it's likely you have some failure in the insulation rubber grommets in the firewall. These are there to pass through wiring and such and maybe one or more a simply dislodged or are worn. This would allow the normal turbo whine to enter the cabin more loudly than desired.
The turbo will kick in at approx. 3K rpm and more quickly under load conditions. To find out if the operation is normal, I suggest the following simple test. Find an open stretch of road, put the tranny in position 2 vs. D, and drive at an engine speed of about 2500rpm. Now accelerate gradually and you should feel the turbo kick in at about 3K.
The other symptom you mention about the whine and loss of power I believe is due to what I said about behavior. "Normal" turbo operation the sound would increase as exhaust pressure rose, such as under load or acceleration. The MB turbo, the whine becomes apparent after the load is reduced and the turbo is slowing down! Struck me as odd, but, I've gotten used to it.
If your car passes the 2nd gear test, I wouldn't worry about it and just get used to the driving charc. of these diesels.