People will tell you that highway miles are better than miles put on in local traffic. The reasoning is that on a highway mile for mile there is much less stopping and going, which equals less mechanical stress. But the real indicator is driving habit.
If you're the sort to slowly accelerate from a dead stop until you are up to speed, you'll put less stress on an engine than if you're the type to accelerate much faster from a dead stop. If you're the type to burn rubber everytime you accelerate, you've got a good chance at eventually ovaling out the bores in the engine block for power loss, start burning a sh!t load of oil, carbon build up, maybe a snapped valve or a bent rod if you're really persistant, ect.
So basically, if you're buying a car from a punk who brags about bad ass rims and laying rubber like no tomorrow, you've got an engine that more than likely isn't going to last as long.
Then you've got the people who will tell you an old lady has always driven the car....just to the supermarket and back for cat food. Whatever. Don't believe it.
And no, you can't reset the engine milage unless you completely rebuild. The registry of motor vehicles will have a sh!t fit.
Everything mechanical wears down eventually. Engines are machined within very tight specs in the factory, often within hundredths or thousandths of an inch and everything has to seal properly in a very high heat environment to be able to efficiently process fuel into power. After a while of use these specs will begin to wear out of tolerance and that's when you get a car that burns oil quickly, loses power, ect.
Sometimes you just need overhaul an old engine withnew piston rings, gaskets and things of that nature. Sometimes you need to rebore the engine block and change to larger pistons. (That's another thing. "Rebuild" is a word often abused. It's more than just a few new gaskets).
All engines can be rebuilt several times to last several generations, but eventually you'll machine the metal down too much to be useful anymore. Most people rather not rebuild, so they let them "die" and just buy new.
Of course, how fast they die depends on how well the car is maintained. I know of people that seized engines on newer cars because they never checked the oil. Then there are 20 year old cars out there with 200,000+ miles on it and still going.