What main things to look for when looking at a 69 valiant?

All of the above, plus where the roof attaches to the upper quarters on each side. There's a seam there that rusts and people often times fill it with bondo. Also, open the trunk and look on each side under where those seams are for signs of leaks and rust. Also, inspect the very bottoms of the quarters behind the rear tires. Look ALL the way under at the pinch weld for rust and body filler. In general, inspect the car down low all the way around.