Engine Light showing on Dashboard for Ford fiesta 2005?

Fiesta 1.6 ....2005 Model : Engine light on Dasboard was showing since couple of days and also the car started shaking and vibrating when slowed down at Traffic Lights. Also it used to take many attempts to start my Car. My Mechanic checked and found that the spark Plugs were all faulty...hence changed the entire set of 4 spark plugs with Platinum plugs and also changed the new Air Filter...... after this the Car starts fine however it still shakes and vibrates when slowed down on Traffic lights and still the car miss fires when slowed down.....also the engine Dashboard light is still on ...Now the my mechanic is telling me that it could be the Coil issue.....hence asking me to change the coil.....Please advise me on this...........I am ready to change the Coil but does it cost around $200 ? and will this fix the issue ? I would highly appreciate your response. Please help me in this. Warm regards, James