I think to say that crash structure rot is endemic to NC's is overstating things a bit, the front and rear bumpers (under the plastic panels) do tend to get some rust on them but I've yet to hear of their actual "crash structure" ergo ability to cope with a shunt being compromised, they are also made of a high tensile mild steel, hence the high pitched ping they produce when tapped with a hammer or similar. Sure they can get a bit rough looking but never yet have I heard of or seen one that has been so rotted that it would crumble in an accident, remember all car manufacturers must design in safety factors to account for ageing and all the associated processes, work hardening, corrosion etc.
I have had my own bumper covers off and yes, there was some corrosion present but nothing that removing them, cleaning them up, applying a good rust protector - in my case Dinitrol would quickly stop in it's tracks. Also the location of the bumper units keeps them pretty dry and protected from salt.
The NCs main area of concern for serious integrity compromising rot is the area at the front edge of the rear wheel arch and behind the door. As is the case with both the NA & NB there is a fairy complex labyrinth structure behind the panel skin in this area, it's purpose is of course to provide much of the strength of the body due to the lack of a structural roof. This area is the main one to really watch.
My own NC was subjected to a full pro Dinitrol treatment at around 3-4 years old and I tend to pop a borescope in various areas, the area mentioned, the sills and a few other sections of the chassis on an annual basis to keep tabs on how things are looking. So far it's all sound but again I tend to load more Dinitrol in there when I do the inspections. For what it's worth a borescope with good enough resolution can be had for under £50 on Amazon or the likes.
In many cases what you see on the subframes and other suspension components whilst not pretty and really unacceptable on any modern car does tend to be surface rust and again a proper pro rust proofing treatment will see you alright for a while to come.
After my last MOT test in November last year the tester commented that the car was indeed "mint" underneath. It may not look the prettiest after the final black wax treatment is applied but it serves a purpose.
Incidentally, the newer treatments should not be confused with old bitumen based treatments that could split and allow moisture to sit in pockets between the treatment and the metal meaning your P&J could rot away unseen, most newer treatments are self healing and actively repel moisture.
I hope this helps.