I've had good luck with Zebralight, which I run mostly because it is unmatched for weight efficiency. For DH, the main selling points will be potted electronics (hopefully improves tolerance to shock and vibe), and ability to safely use unprotected cells (cell protection sometimes blinks off under shock loading). I prefer warmer light, higher CRI if I can get it--though it might be all in my head, I think it allows me to see trail textures better. I also prefer running a helmet/bar combo, which helps illuminate shadows and show me both where I'm going and were I'm looking. I go flood on my bar (aimed to illuminate the ground for about 15'), and spot/flood on my helmet (aimed to illuminate the ground from 10' and beyond). For DH, I'd consider running two floods on the bar.
My setup:
- bar: SC600Fd Mk III Plus XHP50 Floody, CRI 93-95, 5000K
https://imgur.com/UCKLU0H
- lightest helmet (I use most, besides carrying every day): SC63w "warm", 80+ CRI, 4500K
https://imgur.com/ahReRwO
- slightly heavier helmet (better heat dissipation, slightly better beam): SC600w Mk III, CRI 80+, 4500K
For mounting, cheap Amazon stuff has worked well for me--the lightweight ZL's are not difficult to attach and aim.
Best of luck!