PolarPro filters are really good.
Now, in different scenarios you will need different filters. If you shoot in the middle of the day, most likely you will go with ND16, ND8 is good for semi-cloudy, morning, evening shooting or even middle of the day depending of your need. UV filter you already have, it came it your Phantom, and it it for protecting your sensor from UV radiation if you shoot direst sunlight.
I am sure you meant polarized filter instead of UV. Polarized filters are good specially for shooting water since they let in only horizontal light. They will also add nice contrast to your video, and make some small f stop. Means of ND filter is to get your shutter speed to close if not exactly to 2x of your fps. If you shoot 4K@30fps then you want your shutter to be 60, so you can have nice motion blur in your shots. This is especially needed if you shoot 24 or 25fps. ND filters and Polarized filters will keep most of flares out of your shots.
If this is all too confusing, just google for use on ND filters