Rotate to Original Z Rotation

I think that the easiest way to do this is to create an empty object then parent your object to the empty. It should have the rotation set to zero. From there you can rotate the empty in any direction on any axis, without touching the rotation of your object.

I would look into first storing the Z position on "Start()" or at an appropriate time for you.

Second, I would look into

 transform.rotation.z

Finally for speed I would look into

 transform.Rotate(..., ...);

Here are some helpful links for you:

http://answers.unity3d.com/questions/595467/controlling-the-rotation-speed-of-an-object.html

http://docs.unity3d.com/ScriptReference/Transform.Rotate.html

http://docs.unity3d.com/ScriptReference/Transform-rotation.html

All this should help you get started on learning what to do.

Use Mathf.SmoothDamp:

 private int zRot;
 private float velocity = Vector3.zero;
 public float rotSpeed;
 
 void Start()
 {
      zRot = transform.rotation.z;
 }
 
 void Update()
 {
      Quaternion.Euler (new Vector3 (transform.rotation.x, transform.rotation.y, Mathf.SmoothDamp (transform.rotation.z, zRot, ref velocity, rotSpeed)));
 }