Have you seperated the glass section from the runner as this is where rot hides and most of the time you cant see it. Afterwards flip it over, glass down, and then take some pictures, especially the locating tabs which fit into the runner section and the front 2 corners as this is where they rust and rot the most.

If its completely rust free and the glass has no scratches and the seal is perfect its worth £150 all day long.

Dave.