I believe that the NKG plugs are a bit too long and stick a little too far into the cly, or not far enough, I cant remember which someone did a compareson years ago with pictures. Motorcraft plugs are gonna be the best plugs for a DD in the Zetec.
The Timing belt is a fairly easy job. replace everything you can while its torn apart. And since you gotta remove it anyways i would replace the passenger side motor mount as well as they go bad much faster than the other 2.
Good Luck and