lol .. they wont get hot
the numbers (6k & 8k and the like) are the Color Temprature of the light that they give off when running:
4k5 - filament lamp (60w house bulb type!!!), slighty yellow (the "k" is used as a point so 4k5 = 4,500)
5k5 to 6k - white light, about the same as bright natural sunlight
8k - slight blue'y or ice-blue/white tint to it, but not blue
10k and higher - going towards ultraviolet, crap for driving but look good.
In general the
lower the Color Temp of the light the better it is for us to see with, you will see further at night with a 4k5 light than with a 10k light not due to it being brighter or travelling further but the difference between the "light beam" and the "background" is smaller
Also most HID bulbs draw only 30w to 35w - this is "Watt's" and is the power they use, most of this is turned into heat but it's still less that the 50w to 55w of a normal bulb - so if they give of less heat they run cooler.
Legality is a gray area (unlike the lights when running!!!).
** they are illegal to use on a public highway unless the car & headlight housing is designed for them (and if it was you would not need to fit 'em as a mod!!!)
** they WILL pass a MOT fitted as this only checks the Beam Allignment/Pattern and not the actual type of bulb used
As for fitting ..... if you can change/fit a head unit you can do this, took me about 90 mins to do and most of that was working out where to put the balast boxes so they would be secure, on a Leon would take under 30 mins now.
Dave