Code P0125 new coolant sensor

I bought this car 1 month ago, $1600, I thought I had found a winner, and was thrilled that I wasn't ripped off. 1 week ago I drove my daughter to work, came home the car sat for 6 hours, I started it and right away it sounded bad, rough idle, and drove rough It felt like it was going to stall. It sounded and felt like I lost some parts out of it or something was really wrong ! And again I heard or felt nothing wrong with the car driving home that morning.( And the car rattles and shakes, everytime I start it I hear a new noise. A friend took it to O'reilys to run a diagnostic. Test. 1st test was code P0125. We put a new "coolant sensor" in, while he was putting the part on I went on line and read every about cooling sensor and symptoms and it had all the symptons as my car had, new sensor was in NO CHANGE still runs bad. Ran another test codeP0303, replaced air filter, spark plugs, spark plug wires and again no change. What do I do now? And if I drive it what will happen? I have limited funds (on S.S all the $ I had I used to buy the car) Please help!