Hardly bullet proof. There's a fallacy that the PD engine's are invincible. They're as prone to problems as any other modern diesel (granted the 1.9 is a much better bet than the later PDs). There are plenty of horror stories about Ford TDCis, but there are just as many tales of PD engines eating themselves.

Basically buying ANY second hand car is a lottery, and buying a second hand diesel is more likely to be costly (when it breaks) than the equivalent petrol engine.