Bought a 2000 Mercedes-Benz clk a month ago. Car displaced 2 malfunctions.

What is your question?

Head lamps and washer fluid. I did all the changes and a tuneup. The mechanic noted that the serpentine belt needs to be changed and the rear left brake.The problem is the car engine check light is on, I can hear the clutch from the outside of the car when the car shift gears, and the car has problems shifting in all gears. In addition the car is losing speed so i have to drive 30-40mph. What could be the problem? Someone said it could be the speed sensor. Nothing being displayed beside the engine check light