HID sets vary in price, most of all of them come from china, and are very simular from 1 kit to another, but may cost 2 or 3 times as much.
I ordered a good 20+sets of HIDs from www.ddmtuning.com for various people, friends, family, myself etc. Average cost is around 60$ per set (with mounting brackets, wiring harness).
Their raptor style ballasts are bulletproof, their slim ballasts I dont care much fore, and cost $10 extra.
The 5000k, 6000k, 8000k, 12000k etc etc is the temperature of the hid bulb. K means Kelvin.
The higher the number the more blue/purple the bulbs are, BUT the less light output they have.
So a 6000k bulb will be much brighter and more white then a 12000k bulb which is more purple and has less light output. Stock hids in many cars are around the 4800K-5300K range. What is best is your own liking, no one can answer that but you!
To order your bulbs, you first find out what style bulb your vehicle has. Then you goto your hid retailer of your choice and order said hid kits with the same style bulbs.
If your factory lights have highbeams and low beams in 1 bulb, then you normally pay more for the special harness and bulb type. If your vehicle has separate bulbs for highs and lows, you would need to buy and install 2 sets to keep your high beams.
Technically HIDS are ILLEGAL in ALL STATES unless it is OEM equipped with your vehicle, OR you buy projectors and retrofit the projectors into your headlights (steve meade has a video on his youtube channel how to do this, but every vehicle is different but the concept is the same). You could also buy new headlight housings that have projectors, but the cheap ebay kits for $200ish are usually made of plastic, have the potential to melt, and most are made for Halogen style bulbs, not hids, but some have had okay luck!
I have HIDs in all my vehicles for a good 5 or 6 years, and never have gotten pulled over, and dont use projectors. I run 55watt raptor ballasts with 6000k bulbs. I have my lights aimed well and they do not blind others (at least I was never flashed yet, or pulled over for it).
HIDs take more power for the initial start up (the first 10seconds or so, when the ballasts are warming up before putting out their full light).
After the ballasts for the hids are warmed up they draw less current then your factory halogen headlights. You will also not notice the hids dim like normal headlights do from the amps sucking power to make bass since most ballasts operate in the 9volt to 16volt range to achieve full output.
I highly recommend using the prewired relay harness that you can usually buy from the hid retailer, which will make the headlight suck the juice from the battery, and not through your factory headlight wiring. In some but not all vehicles the factory wires can not support the needed amperage for the initial turn on of the ballasts, the relay harness fixes this.