Longevity is all in the maintenance. If the previous owner has all the vehicle maintenance history and it looks like things have been kept up, especially oil changes, then it will be a worthwhile buy. It's when you can't get any records on a used vehicle, that's when to start asking questions and even walking away.
The 3.6L V6 is a good engine and if properly maintained you should see close to 200K if not more depending on how you drive. The 2wd variants have a lower final drive so cruising at 60mph the engine spins over about 1600-1700rpm, which is decent, but if you want to pass anyone you'll have to put the pedal down and downshift a couple gears as the 3.6L power and torque are both higher in the rpm range. For a highway cruiser and occasional people hauler the Acadia shouldn't disappoint.
