Given proper care and maintenance, a diesel engine will last practically forever. It's the OTHER parts of the vehicle that usually go first. My 6.4 liter PowerStroke has over 220.000 miles on it. The only things I have had to replace are the thermostats and the radiator. The latest oil analysis from Blackstone Labs shows the engine in my truck is in fine shape with no indications of abnormal wear. So the adage that you have a big repair bill at the 200K mark doesn't always apply. Look for a vehicle that has obvious signs of being well cared for, such as no drips or leaks underneath, a clean engine compartment, gently used interior, and a sharp exterior. An owner who takes care of their vehicle will take care of those things, and an owner who drives it like he stole it and could care less about the maintenance issues will not.