Hey Mark....you asked an interesting question. Keep in mind that just about everything on a car was designed with certain mileage expectations. I once saw a chart (wish I could find it now) that listed age/mileage expectatations for just about every component under the hood. From the starter motor to the alternator, water pump, power steering pump, etc... So even if you get 400,000 miles out of the engine and transmission, there's a good chance that other components will fail somwhere along the line. That said, I've seen plenty of vehicles (Toyota and others) go more than 200K with nothing more than normal maintenance repairs (brakes, oil changes, timing belts, etc...)

Now if you're looking at a 4th gen 4Runner and you're concerned about maintenance, keep in mind that the V6 engine has timing chains and the V8 has a timing belt. You also mentioned spark plugs....the V6 plugs are scheduled to be changed every 30K and I believe the V8 has iridium-tipped plugs that are good for 100K. Then again, you could always put iridium plugs in the V6 and not have to worry about the 30K plug changes anymore.