For M mode. Use Auto ISO. The D7100 will still meter and change the shutter speed and ISO depending on the scene and the aperture setting. I set Auto ISO to a min shutter speed of 1/30 and max ISO to 3200. On a bright but cloudy day I set the aperture to f8. My shutter speed varies around 1/125 to 1/160 and the ISO moves between 100 and 200. You want to do some experimentation before you take an important video.
On my DX bodies I like to use an AF 24mm lens. One thing to be aware of. The built in mic is sensitive. On a breezy day you'll get a lot of wind noise.
I generally don't shoot video that much so someone with more experience in this area should chime in with their recommendations. But I wanted to get you started with something that works for me.
My D7100: -Photo live view in M: I need to press "info" button a few times to get exposure meter -Video live view in M: no exposure meter. I only get sound meter, grid and level meter when pressing "info" button.