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Start with the P0440. Fix the large evap leak first. Could be as simple as the gas cap or a large vacuum leak. Fix it, clear it, make sure it doesn't come back.
Then I'd move on to P0171. Follow the service manual diagnosis for that one and determine why the air/fuel is out of whack. It's too lean for some reason. It will have you monitor the front O2's, check for vacuum leaks. Vacuum leak very possible on an older vehicle, also lazy 02's if they are the original ones. Clogged injectors possible... that often gives very poor cold start, and hesitation problems.
Dig into P0420 last. That is for the cat converter efficiency. I'd get it running correctly first so P0171 and P0174 don't come back. Something is making it way too lean. Once it's running right, clear everything and see if P0420 comes back. Then you'll have to determine if the cat conv. is actually bad or just a bad rear 02 sensor.
Clogged injectors are possible on something that old. Some injector cleaner like Chevron w/Techron in a tank full of premium wouldn't hurt. GM top engine/injector cleaner is also good stuff, has the same Techron cleaner. Change the oil after that tank of gas and cleaner.
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