1997 trouble codes

Hi, I just bought a 1997 taurus wagon with the 24V Duratech engine it has a rough idle and shutter when at a stop (unless I put it in neutral) and when first pulling away untill the rpms got higher that made me think it could be a loose motor mount or transmission problem because it has 166,000 miles on it. Also the check engine light would come on and off intermittently. I checked the transmission fluid level and it was very high. With a hand pump I removed over 3 liters of transmission fluid to get it to the top of the cross hatching when ideling and warmed up. The next day which was to day a friend came over with his OBDII scanner and the car gave the following codes
302 for number 2 cylinder miss fire (the previous owner just put in new plugs and wires should I pull and inspect?) is in the gas tank injector cleaner the first thing to try and which one? and 171 for system 2 lean bank 1. Is it likely that i have bad O2 sensor and which is bank 1. how are cylinders numberd when facing the passenger side looking at the serpentine belt? Should I be wary of driving it if it is running lean or does the computer give a code at just a little lean?

Thanks in advance

Jeff