Back to the original question..." What ND filters do I need for shooting sunsets?"
In my opinion, ND filters are only useful to control the amount of light reaching the camera lens and are mainly used when shooting video. So if you are filming in very bright daylight, the camera will need to be set to higher speed/minimum aperture to achieve correct exposure. However the rapid frame exposure tends to look quite "jerky" rather than the smooth motion we like to see in movies. This is where Neutral Density (ND) filters can assist by reducing the amount of light entering the lens and thereby allowing a much slower shutter speed (ideally around 24 frames per second) while still achieving correct exposure.
Your video will look smoother when filmed at lower shutter speeds.
So in answer to the original question, I don;t think there is any need for ND filters unless you are shooting video, and you need to reduce lighting in order to achieve correct exposure at a slower shutter speed. Unlikely scenario for sunset photography.
My thoughts only,
Cheers