Engine is overheating?

I just purchased this car less than one month ago from a used car dealership. The check coolant lever light kept coming on. Finally five days after I bought it, it started overheating. I took it back to them and they said they took the thing that holds the coolant out and put in one from another 2006 CTS they had on the lot (they also said they did the same thing with the radiator and the lines, I know that was a lie) the car did stop overheating so I thought everything was fine. Then two weeks later after it snowed I noticed a huge puddle of coolant under my car, the light has not come back on but now the car is shaking while in park and while driving at a low rate of speed. So much to the point that I turned it off because it felt like it was going to go out. The check engine light came on the night I bought it and the oil said it had 12% life left, now it says 0% is that possible within a month if there is nothing else wrong? Please help I know next to nothing about cars.

When I turn my A/C on the overheat light comes on. I see no leaks.