Nails should ALWAYS be set before sanding a floor. And filler added to the holes and any other imperfections.
Remaining thickness is hard to determine unless you can get to an edge or cutout and actually measure the boards. It isn't the power of the sanders, but the tongue and groove flooring that determines how much you can sand.
Once you get down to the groove, if you go too far, the top edge of the groove will break off and/or expose the tongue underneath. Not a desirable look and only way to fix this is to add a new board(s).