Wow - that is one rusty sill!
I had an FD1600 as my first car, then an FE1800 and FE2300 after university. The original outer sill became so thin that I gouged a huge hole in it against a rock in a car park, but the middle part of the sill was still perfect - that would have been in 1987 (on a 'N' reg car).
The FE1800 had a good body, and the FE2300 a good engine so the way forward was obvious! I loved the FE much more than the FD.