I'm reading it as he has a zetec, but is more aware of the problems with the SPI (since they are more publicized).
Neither engine, and this also includes the Focus Duratecs, have problems with head gaskets.
Anything you may find that says otherwise can be attributed to installation error (flatness, bolt TQ, surface cleanliness), user error (overheating), coolant degradation or type and other things.
In stock untouched form the gaskets are very good.
Lets say you buy a car with high miles, coolant has never been changed. If the head gasket fails we can blame the coolant. Over time it does become corrosive. Left long enough this will eat metals.
If proper maintenance has been done, there should be no issues at all.