1985 300d turbo

[kaleope]

not much experience here, had the car 6 months and I love it. Car would not shut off so we by passed the power door locks and it worked ok. Now it started to not shut down again except for using the kill button under the hood. Brought to the mechanic and had belts & oil changed. Replaced brake booster. Get the car back and it wont go over 40 when floored. Some odd smelling smoke comes out at times, like a fiberglass smell. Other times black smoke at full throttle. Rougher idol than before. I returned the car to the mechanic and he says the turbo is no good. I am reluctant to change it because the car ran ok before. Always slow and not the mileage that everone talks about but ok. I miss the car, any ideas ? (120,000 miles)