Lol if your planning on using it on the road 12,000K is asking for trouble. Headlights have to be yellow to white in colour. Any other will not pass a mot and on top of this is illegal. Also HID retrofit bulbs are illegal in general.
The best for light (as in gives out the most amount of light) is general down in the 4000ks Normally around 4200 to 4500. But if your talking best quality light to see by in the UK many people would say light around 3000 is best as in rain, snow and fog as it gives best visibility.
Also just so your warned yes those lights in the Ebay link are illegal. Also I would suspect the warranty is a joke. Not it says manufactures warranty but they don't even advertise the brand. Good quality HIDs come from OSRAM, Philips, Piaa and a couple of others but not many. But be warned there are many poor copies about that are boxed up as a good brand but are far from this. Most from ebay are just poor at best.
Have a look at
www.hid.moonfruit.com