1. RUST. And lots of it.
Look under each door, where the door skin mates to the frame on the very bottom. Look at the edges of the trunk lid or hatch, esp underneath where it sits on top of the tail lights. Look on the rocker pannels all the way down the car. Those are the biggest rust spots.
2. Rear main seal leak.
Look ALL over the underside of the motor for oil. Don't be alarmed if there is a few drops, they all do it. But if there is a LOT of coating or significant puddle under the car, that's bad.
3. You have the weakest 302 Ford ever made. IIRC it's something like 145HP and 175TQ but someone who knows for sure can chime in. It's carbed which sucks donkey balls. MPG? If you have to ask, don't buy the car.