I bought my Nav 2nd hand.
I've done about 100 000 km in it. I and everyone else who sat it it thought it ran really well. Well until I cooked it.
As I pulled the engine apart, I was amazed at how bad it was.
Not just the damage from overheating, but what a "mechanic" had done previously. I reckon the previous owner blew the head gasket, their "mechanic" unbolted the turbo, lifted the head about 2 inches, pulled out half of the de-laminated gasket, slid in a new one (crooked), and used a rattle gun to wind the head down until it sealed.
Then it was sold to some sucker.
I spent a few months after work replacing everything above the block.
No rattles. No knocks. And it pulls like a teenager.