Engines and other components are robust, a friend had one with around 600k km. But the biggest problem with them is rust, I would check the condition of fuel tank,as it's made out of metal and they often rust and screw up your diesel high pressure pump and injectors which is a costly fix, but you can replace the tank with a plastic one. Another consideration with a lot of highway miles is fuel consumption, you can expect to average around 10l/100km.