The 1HD-FTE is a very reliable engine as are most of the 1HD-XXX family of Toyota engines. The single most important thing is to get evidence of regular engine oil and engine oil filter changes. Both should be done at 5000 Km intervals. Otherwise the timing belt is due at 150 000 Km's. Contrary to popular belief, this engine is a mechanically injected diesel however the fuel timing is electronically controlled. In Australia, most diesel experts believe that it is the best factory turbo diesel engine released in the Australian market. The new 4.5 V8 Turbo is giving it a run for it's money but still considered "new" to the diesel engine scene. Look for a model with full service history and you should have yeas of trouble free motoring out of the 1HD-FTE engine.
Cheers,
P