First off i have had

first off i have had nothing but problems with my 02 chevy z24. First paid 800 dollars to a dealership for the security light and theft light coming on. Turned out it was an after market security alarm someone else that owned the car prior had put in. Next i have nothing but problems with my check engine light coming on. First it was the coils and spark plugs were shot. I had all of the replaced. Next it came on again and i had the o2 sensor replaced. Now there is a loud clucking noise when i drive down the road. It feels like it is coming from the front end. After i got the o2 sensor replaced the check engine light came on again!! I dont know what else to do. Can you help me out with this???