Welcome to BLF
That is the exact same setup I run on my Downhill bike on my street bike I run a single C8. I have both my bikes and my brother has his setup with KDC8s and this mount.
http://kaidomain.com/ProductDetails.aspx?ProductID=11097
I have bought at least 6 of these mounts all good.
http://kaidomain.com/ProductDetails.aspx?ProductId=5255
Review of the KDC8
You really only need one KDC8 on low or medium setting unless you are going mach 9 than 2 lights on medium or one on high is more than enough light and don't forget a bright taillight so you don't get clipped from the back. I always carry a spare battery in my pocket and a backup light just in case, but I was a Boyscout.
Batteries and charger.
This is a pretty popular charger on here I have bought 4. I still have one I use and all the others I tested and sold all work good still.
http://www.dealextreme.com/p/trustfire-tr-001-multi-purpose-lithium-battery-charger-12594
I have over 20 of these batteries they work great in the KDC8.
The Trustfire 3 XML is a great light, but I never use anything but low and sometimes medium when riding with it it is a little bit of overkill.