generally undersides are ok, its the arches that go on them and the sills on the old mk1's.
also depends where you live or where the car has been most of its life, a car thats been kept down devon with the salty air will obviously corrode alot quicker than a car thats been kept in birmingham.
either get a big tub and brush it all around the inner arches and maybe inner sills if you really want to, or if you got an airline get some graviguard and spray the arches and sills. could even get some waxoil and do your inner arches with trims off if you wanna go hardcore on it