This seems to be a common 156 problem. Mine had what seemed to be very dodgy sills and several sections of the floor, (when viewed from underneath), were unprotected, i.e. no underseal, yet surprisingly proved no problem at MOT time.
Closer inspection showed that this was indeed just surface and looked far worse than it was. Structurally it is still sound despite being a 1996 model with 115,000 miles on the clock. As advised above, a good wire brushing and some paint-on sealant means you CAN sell with a clear conscience, it's only routine maintenence after all, just as you would wash and wax the body if you were selling.