OK, for O-80 I would plan on 8 feet of platform width for a 180° turn. This gives you play room no matter which track you use.
If you want buildings outside the track, perhaps a bit wider or have the track swing in away from the edge after the turn.
For curves this wide I strongly recommend around the wall benchwork, with ample walking space and reach of less than 40" to the walls.
Make a lift or swing out section across any doorways.