Thank you for the VIN. We did not have the car in our Club's registry, as expected, I must admit. All the VIN says is: 2500, manual, LHD, ECE, Series 3. I did a quick research on the half visible reg. no., it seems an original Dutch car, first registered 22nd April 1976. It has passed the annual technical check and is currently road legal. Power steering may well be, sunroof could be or not, a/c would be an immense surprise.
Your question: Diagnoses from far away tend to be inaccurate. Anyway, rust means there was water. I'd say it came past the windscreen sealing. The rubber may shrink over time and make way just a little around the bottom corners. Even if the top corners of the firewall were badly rotted they'd be very unusual spots for water to come inside.
The third picture I don't like too much. The circular hole in the spare wheel well served as drainage for the primer paint, the bodies were dipped completely in a basin. Four more are in all footwells. The holes were closed after priming by plates with bayonet locks. What we have here I don't know. Furthermore, like nearly all cars in the Netherlands, this 2500 had a trailer hitch retrofitted, the bar painted over black. Cannot think of any other explanation for the piece. The rear of the E3 (E9 as well) is quite soft. Pulling trailers was not something the Munich engineers gave a lot of thoughts to. In short: body structure does not like it.
One good thing. Note the little black plug where sill and A-pillar meet. The car had cavity conservation when new. This surely has slowed down corrosion of some parts.