HD should do fine, but the audio will be terrible from my experience.

You will need fast memory cards.
Not just fast in bursts like you need for stills.
But fast sustained speed, and that's a very different thing.
I have some cards that are fine for stills, even with RAW images, but in my wife's Everio video camera it doesn't record smooth video.

So look for one that matches the video camera.
I discovered I should have bought class 6 memory cards.
So now I have 3 more cards for my digital camera
And need to go shopping for some cards for Angie to take videos.

I would expect a video only system would be better than a still camera with video.

I use Culmann mounts that are a little simpler than yours, and with a small ball-head do well for in-car video. I also have clamps for on-car video mount so I get the track as well.
I use an old Minolta A1 and Konica-Minolta A2 as sacrificial cameras, but plan to borrow my wife's Everio (small enough to fit in a pocket) to try next time and put the A1/A2 back on stills.