I'm more of a G body guy but did own a 70 Impala quite a number of years ago. I guess my first thoughts by just the age of the car would be rust and not knowing where these cars rust I'd have to say check the body mount braces, frame, floors that kinda stuff.
I was always one to watch for as many options as possible. I'm a huge fan of tilt wheel, pwr windows and locks. Years back I had a buddy that bought a 67 Impala 4 door hardtop that had factory air, tilt and pwr windows. The car also had the 275 horse 327 in it, was a pretty decent car.