Fix the AC and oxygen sensor. The O2 sensor should be a really easy fix. But if the AC needs a compressor, then but the least expensive compressor you can find. Hell, buy a working USD one if you can find one. In the summer, NOBODY wants to buy a car with no AC. Replacing the AC compressor actually isn't even that hard, but you need a vacuum pump and manifold gauges to properly recharge the system. Most shops will do it for you if you bring it in after you replace the compressor yourself. But if you get the whole job done at a shop, be prepared to spend at least $1200-$1500. I did mine myself for around $550 total, and that was with me buying a brand new AC Delco compressor and drier, the orifice tube, PAG oil and R134, new belt (both belts actually) and tensioner, and then paying a shop to vacuum and recharge the system.