It often isn't possible to inspect the inner hull. Many (I'd hazard to say most?) boats don't have easy access to large areas of the inner hull.
The external ultrasound though will tell you the thickness of the "good" steel, irrespective of where any rust has removed metal. So a reading of 9.5mm will take into account any corrosion internally or externally - so by the sounds of it, you don't have much to worry about on that front.
You've got an advantage of being able to talk to the actual seller here, so ask any questions you may have to him directly, then just instruct the broker on what you want done - at the end of the day, they're making a lot of money out of just selling the thing, so they can do as they're told!