I think the largest reliability problem would be the atx transmission attached to either motor. The atx in the zetec is slow so it will abused to anyone without the patience for the car to accelerate. The atx on the duratec will take some spirit driving, but its still an atx and will die under abuse.
The two blocks both go for a good longevity with regular maintenance. Rarely do you ever see a post about head gaskets, bad/bent values, falled bearings, etc. I'm not really sure the weakest points on the zetec, but the duratec would we the evil long right handed turns at high speed causing oil starvation. But I have seen that maybe twice on ceg? You really need to on the car in a right hand turn to do this.
The regular aging car issues apply, alternators seem to be a big one. The water pump with a plastic impellor. Usually see some AC issues. The sunroof gear has a fault with the worm gear as well. BUT you need to remember the newest model would be 2002, if you find it from a state auction. Most models are 2000 back so your looking at least 6 years old. So most problems are general problems with any car.
