Hi fejji, the hose at the valve cover will not fix the engine running. If the car has a top speed of only 45 mph I would replace the fuel filters and inspect the screen inside the fuel tank for algae contamination. The major service should include a valve adjustment, air filter and fuel filter replacement and an oil and filter change. If he's including additional fluids such as coolant, brake fluid and transmission fluid the price sounds very reasonable. You may want to wait on the major service and focus only on the performance issues. The wastegate hose to the turbo should also be checked but it won't affect the top speed. Let me know if you have any questions.
No, the hose won't fix smoking out the tailpipe. The fuel screen is attached to the hose at the bottom of the fuel tank. The tank will need to be drained first and then the screen unscrews from the bottom of the tank. It helps to have the special socket to loosen the screen with. The wastegate is attached to the turbo. The hose is the small 3/8 inch diameter hose that connects to the wategate. It needs to be checked for leakage. This website advertises Mercedes manuals. I've heard mixed reviews. You can usually find the genuine factory repair manuals on eBay.
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