I like flying early mornings and sunset if that can help you give me a better idea of which are good for high UV rays?
You'll normally need to block UV when you have a lot of diffraction or reflection, for example in the morning if there is a lot of haze in the air, or if you're shooting over water. You don't really want to use a polarizing filter if you don't need to, it will alter the colors a little.
I think
@Richard R is correct that ND filters do not inherently block UV. The Polar Pro set I have has both polarizing and non-polarizing for each stop (except the ND4), for example there is both an ND8 and an ND8/PL. I first picked up the "6 pack" set (
DJI Phantom 4 / Phantom 3 Professional Filter 6-Pack ) which should cover you. The "3 pack" set seems to be only way to get the ND4 right now, which is a little annoying since the other two are duplicated in the 6 pack.
Mornings and sunset are tricky. It depends a lot on the angle at which you're shooting. If you're into the sun, you might need a higher stop. Away from the sun, and a higher stop might not let enough light in to catch what's in the deeper shadows. So, it's a trade-off, and I'll often have to try a couple out for a given situation before I get it right. And by then, of course, the sun sets and the shot is gone.
The other important thing
@Richard R points out is that by keeping the ISO very low, you can keep the graininess of the picture minimized. By slowing your shutter speed, usually to half your frame rate (I normally shoot at 30fps with a 1/60 shutter speed), you get just a little motion blur which gives that nice cinema/film quality. With the shutter open that long, the filter is what stops the sensor from over-loading (since we can't directly control the aperture on the P3).