My 328i has lived in Maine from new (much worse than NJ), but I only drove it in winter the first two years I owned it. The only rust is the various screws that hold the underbody panels on. I bought a bag of new ones to replace them, just need one of those 'round tuits' to get it up on my lift and do it. It's major service time again, so probably next week while I am on vacation and draining all the fluids anyway.
The only other real issue is that even though the car was only driven in winter for two years, the lingering sand on the roads has pitted the headlights pretty good, though they have zero clouding (always garaged). My 128i was a Texas and now Florida car, and the headlights are all but pit free, as is the original windshield. The windshield on the 328i was getting pretty badly pitted, but a few years ago it cracked while in winter storage so it got replaced at the dealer (very glad I have good insurance, THAT was not cheap, but you can't tell it was replaced). Perfect now!
I guess one further issue is that the salt the 328i did see caused all of the aluminum bits to have surface corrosion. But that is purely cosmetic. But the 128i looks nicer under the hood because of the lack of that. I just keep the hood shut.
