Yes, get the spot attachment - is better than none. Yes, get 2 more backs at least, esp. if you shoot a lot under different lighting situatuion. If it's all studio work, forget the spot and extra backs. With an incident meter or a non-spot reclective meter, you'll need to get close in to the subject to meter the range of luminances.
It all works so long as you can meter the different areas of the scene. An overall average reading won't work with the ZS. Of course, if the 'dark' are of the scene is on the other side of a valley with no bridge, just meter something on your side of the valley that is approximately the same reflectance - or jump.
The Luna-Six with teh variable spot attachment is ok. I have one and have used it but went with the Pentax 1Deg. Digital when I could. Your 5Deg attachment should serve you well for a while, I'd think.
Don't forget to calibrate it with the ZS tests.