That things been averaging close to 26k per year! Well over the norm. I wonder if it ever gets a chance to cool down? While I would agree with some that highway miles are better than city miles, there's an important thing being ignored, the pistons have gone up and down, many, many times. There's no way of getting around wear and tear highway miles or not. No one has mentioned checking cylinder compression and that's where I would start. And that's after I've gotten a fair price negotiated.
IMHO you could find a better deal with far fewer miles.