Thanks !
I asked for RAL 7005, it was the closest color in the RAL palette.
The most rusted area was where the fuel tank closing plate sits, with some light surface rust.
The grooves in the sheet metal, which are supposed to let water drain, were clogged with dirt, so water had accumulated there. It was rusty over a few millimetres.
There’s also a tiny bit of rust inside some closed sections of the chassis, but nothing serious.
When I bought the car, I was expecting damaged body panels (wasn’t disappointed!) because of the paint, but the chassis looked solid in the photos. So no surprises on that side (only on that side, actually).

- tole fond.png (612.9 KiB) Viewed 58869 times
Cheers