Forget the mileage.......period.
Tell the seller you want to start and drive it from stone cold, i.e. first start of the day. You will then start to get a feel for how the car really is, the gear change goes through an evolution process as it warms through. Difficult gear selection not uncommon but, if a little stiff when cold but improves when warm then that is about as good as it gets, lot of BS hype out there on additives. If the engine smokes when cold and does not clear then that could be expensive.
Look at the knuckles on the axles behind the wheels, grease running out from them is not good sign, not massively expensive but try driving in circles on full lock and listen for the CV joints, any noises and now it is going to start to get expensive, in particular if you are going to rebuild it to OE spec.
Now look underneath for rust, the known places have been mentioned, Rust is expensive to repair properly.
Now to the interior, good clean interior parts are expensive to get unless you are super lucky.
Final thought, look at your budget then at the prospective purchase. If the cost of buying the car and the projected costs of repairing exceed your budget then walk away. I say this because your guess at pretty much any repair on an 80 will be woefully under estimated if it is to be done properly.
Mine has only 240,000 + miles, just this minute got back from a 260 mile drive, not a lot true but it is as effortless today as it was 5 years ago when I bought the car with 182,000 miles.
regards
Dave