follow along the XZ plane?

Hello. How can I have an object follow another object along the ZX plane (not changing its Y position at all), but have it stop when it gets within a certain distance? This is what I have so far:

var player :Transform;

 function Update(){
      transform.Translate(Vector3.forward * 10 *Time.deltaTime);transform.LookAt(player);
 }

(if it helps put it into context, I am trying to get a ship to follow crosshairs in a starfox 64 type game) Thanks