major problems with my car!!

Hello, I had recently bought a 1996 Chrysler Concorde with a 3.5 engine. It has 151,200 miles on it. I bought the car because my sister needed the money to buy another car. Okay here is what happened, My sister was letting one of her firends borrow it. They never checked the oil and her friend was driving it and all the sudden there was loud noices from the engine and it wouldn't idle smooth. The oil light came on and it never started again. I bought the car because it has a great body with no rust or dents and she gave me a good deal on it. When I bought it I checked the oil and it was about 3 quarts low. I put new spark plugs in the engine and I got the car running again. But it does not hold a steady idle and the engine makes loud noises It almost sounds like a diesel truck. When it is running it idles around 1000 RPMS and drops suddenly down, then it will go back up. When I put it in drive or Reverse the engine quiets down but when I drive it there is no acceleration it will go upto about 10 miles per hour the the car will clunk out. I have also noticed the Check Engine light bluinks like it is givving a code. It will blink 2 times stop 2 times stop 1 time stop 5 times stop and blink 5 more times. 2 2 1 5 5. I don't know much about the 3.5 engine. I did check the timing belts and it is still in time. Do I need a new engine or could I just get parts and rebuild the parts that are not working? And also, after I got it running I think the engine may have overheated. The engine hasn't turned over in the past 2 months. The engine cranks but it will not start. Could it be the timing belt? I don't really know how to check the timing belt, it seemed tight. I don't know. If the timing belts to jump, does it do any damage to the engine?Please help.