Considering an aftermarket conversion or trying to see if you can upgrade to HID? First consider the overall costs of piecing together an HID system.
High Intensity Discharge systems are expensive and 1.consists of a non-filament bulb filled with a noble gas, such as Xenon, that are affected by the temperature range. Kelvin is the standard by which the hue or color is determined, the lower the K(4300-5000) the whiter the light and perceived brightness, and vice versa, the higher the K(7,000)bluer to (12,000)violet/purple, but lower perceived brightness. Because human eyes work better in daylight, due to the fact that we have more cones than rods, cones function better towards the yellow-white range. So if you really want a bright HID go for the low numbers. 2. need special wiring to deliver high voltage to start the arc/discharge. 3.require a ballast and an igniter. The main cost of a system comes from the ballast. This is the coil that transforms 12Volts from the battery/car electrical system to the needed 12,000Volts to make an arc. Older generation ballasts were fairly large, bulky and got hot to touch. The newer gen models are slimmer, smaller and some are finned/ribbed to allow heat to dissipate. Bi-Xenon ballasts are more technically advanced, that they allow the same 12,000V HID bulb to operate with the high beam 12V Halogens.
As far as bulbs go, basically there are two types in the older generation HID systems, D2S and D2R. D2S bulbs are clear, while D2R bulbs have a ceramic sleeve coating. Projectors, basically a  magnifying glass in reverse,  take the random light output of a D2S and focus the light using the lens, while a D2R utilizes the parabolic effect of the mirror housing, while the ceramic sleeve limits head-on glare.  Make sure your housing/bulb socket will accomodate the special flange of the D2S/D2R. Various manufacturers have different means to ignite the arc. US car makers often have bulky igniters built into the socket while Europeans use smaller igniters. Some of the very old systems use a separate igniter and ballast, resembling two packs of cigarettes connected to each other. In piecing together a system, avoid buying different parts from different makers, as the slight differences in efficiency between the makers can ruin your electrical system. Try buying in pairs or from the same seller who is parting out his/her wrecked car. Avoid cut wires if you are buying reclaimed parts, intact sockets/fittings often mean the seller is genuine and pays attention to detail. I've seen a wrecker/salvager take apart a car to a million pieces in a  few minutes, using only a wire cutter and an impact wrench.
Most of all, note: different OEM makers have special fittings for their brand of cars. Japanese electrical sockets although metric, don't always fit Europeans and especially often not, American made cars.