Power steering issues? Electrical Issue? What is the root cause of this? 2003 Ford Tarus?

I bought a used car about 8 months ago and have been having issues with it. I'm going to start at the very beginning and give as much detail as possible. First bought the car, owner said he put a new battery in because it'd been sitting and wouldn't start and that it needed a new water pump. My uncle (self employed mechanic for 30 years) replaced it for me.
Was running fine until summer hit and I started using the air. If I used max A/C and drove for more than 20 minutes then parked my car and let it sit for 20 or more mins it would not turn over, it'd just sputter or start up, sputter then die, even while giving it gas. I'd have to sit and wait for it to finally turn over and once i started driving it was fine. Took it to my uncle and he said it was the starter so he replaced it. **Note: i had no other issues before this moment.**
When I got the car back nothing initially seemed wrong. I drove it for awhile, under a month, and my gas gauge started having issues. Sometimes it would work, sometimes it would read more or less than what was really in the tank. And it doesn't matter if the tank is full or empty. The check engine light came on a day or two later and i had it checked but i don't remember what code it was throwing.

Part 1, continued in comments

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