1994 Nissan Sentra engine/oil leak

Recently I acquired a 1994 nissan sentra with 218,000 miles. I just replaced the exhaust (that costed me over 900 dollars) and I realized there may be an oil leak. When I shut the car off there is smoke coming from the engine. There is a fluid, possibly oil around the upper part of the engine and along the left side near the alternator. The last few days the car seems to be running hot. Could the over heating be due to an oil leak? The heat in the car does not get hot either. Even if the car has been sitting all night the temp gauge says the car is warm as I start to drive the gauge rises and the next thing I know it is at "H". I turn the heat on full blast to try and cool down the engine and the heat finally works. Well because the car is going to over heat of course. What is going on? And another thing when I try to make a turn either left or right it feels like when a car stalls out and you cant turn.