Most did not have the rust problem so the odds are in your favor. My 2008 went nearly 300,000 miles and 10 New England winters with no rust at all. Other people in much milder conditions had serious structural rust. Go figure. My guess is that some Patriots got a lousy alloy; Just recently someone on this site suggested that the rust was related to how the a/c drained. I have no clue, just passing on what I read on this here grapevine.
Some did have the subframe rust, but it is repairable and not cost prohibitive. I think there is even a youtube video on it (memory says it was a Dodge Caliber, but those are built on the same platform so its roughly similar).
If you have doubts get it checked. Even get a second opinion. You don't want to ignore a potential safety problem, nor do you want to get sold a repair you don't need.
Eventually anything will rust. There was a frame from a 1930ish Ford on my former property that was barely recognizable when I bought the place. In the 20 years I lived there it virtually disappeared due to the rust.
