The corrosion recall affects both the QX4 and Pathfinder. It's not just an east coast thing, but the folks that live in salted states naturally get hit the hardest by symptoms.
I'm fairly certain the QX4 didn't have softer suspension. When I was looking for replacement coils and struts for mine, manufacturers generally specified the same part for both vehicles. I can't speak to the sound deadening but I doubt it. Mine isn't noisy, but it's no Cadillac.
The rear main seal is another thing that wears out over time, but that's a pretty universal car problem. The upside is it failing generally just means a minor oil leak. The downside is that it's cost prohibitive to replace the $10 part. I got lucky (?) and had mine replaced while the transmission was getting fixed.
rgk makes a damn good point about the power valves. Unfortunately there's no easy way to check it, but make sure it's at the top of your list to check after you buy, regardless.
Double-check the codes a prospect is throwing. Some are super simple, others can be clues of things like an ingested power valve screw
