My previous truck was a Chevy Diesel Duly 3500. Not a thing wrong with the engine or transmission after turning 120,000 miles. But the fat fenders were made of fiberglass and had really bad spider cracks in them. To replace them meant replacing the entire side of the truck bed and at $4000 a side, that was just too rich for my blood.
The doors started rusting out at the bottom. When inquiring about repainting, the body shop would not do it; said the new paint would not stop the rust inside the doors and within a few months would each through the new paint and they would not be liable for a faulty paint job and declined to paint for me.
So we traded at a dealership and still got $18,000 in trade in value, which surprised us to no end! Dealer bondo-ed the spider cracks, slapped some paint on the rust and sold it for $30,000! I imagine that truck is still on the road, strong as a Clydesdale Stallion! But I bet that body is near gone now!
Parts fall apart LONG before the engine and transmission! With good upkeep, a diesel should last all your life! If you don't mind paying the price for body and parts to be fixed!