You are right cars rust in different places. My parts 420SEL was rusted in some strange areas! The metal strip between the windows on the rear doors? How the heck does that thing rot through?
My solution is this:
a. By a rust free car and keep it clean by washing it and hope for the best. IE to delay the rust long enough to get your money's worth out of it.
b. Have a nice one for the salt free months and another for the salty months.