Which O2 Sensor needs to be changed on my ford contour?

I got a check engine light on. I did a few things to my engine and none of it worked. The last thing that needs to be done is the most expensive fix. Of course that is last. But my car runs fine until it heats up and then the check engine light comes on. Now i got 2 problems with my engine and i cannot distinguish the two on the order of importance. I have a 2.5L V6 standard Manual transmission. This 5-speed can pick up speed pretty well. But as i drive it once the car heats up it has a hard time idling down when i press the clutch. As the RPM goes up when i get ready to shift once i clutch it the car RPM stays near the shift point. Same as for revving the car in neutral. I'd rev it up and the car takes years to idle back down. My guess is the Downstream O2 sensor considering i think it has something to do with the exhaust. I also have a belt tension problem with my AC. The ac doesn't work and once the car heats up it makes a whining noise then shuts off sometimes. The only way to get rid of the noise and prevent shutting off is to drive. I'm also wondering if the idling problem is coming from the belt being restrained by the AC. My question is, does this engine affect has to do with my downstream O2 sensor or my restrained belt. Otherwise i know that the O2 Sensor is broke because of the poor vibrating idle sounds it make and the poor fuel mileage. Also my car begins to lose power after it heats up. It doesn't a lot of power but when you drive your car a lot you can feel that power struggle that it has. That's the most important part. I want my Horsepower lol