I agree completely with the fuel burned being a better measure of wear than strictly engine hours! After all, higher RPM uses more fuel for the same hours.
All things being equal (same engine, age, maintenance, etc.), the engine run at 1400 RPM will generally experience less wear than the same engine run the same hours at 2400 RPM.
For example, the engine run at 1400 rpm for 3500 hours will have 294,000,000 revolutions while the engine run at 2400 rpm for 3500 hours will have 504,000,000 revolutions. (A difference of 210,000,000 revolutions). For general wear and tear, it is obvious that the engine with more revolutions should have experienced more wear. This is obviously a simplification and does not factor in "other factors" like engine temp reaching the recommended level, etc.
However, for this discussion, as stated by myself and others, having 3500 hours on a 20 year old engine is not really high hours. The buyer would still be wise to have it checked completely by a recommended professional to better understand the actual condition of that engine.
Edit: The friend I mentioned in Post #18 who has over 10,000 hours on his Cummins 6BTA normally operates his engine at approx. 50% throttle which does not work the engine hard, but does work it enough to operate at the proper temperatures, etc.