There are two ways to do this as far as the retrofit to the oem headlights. 1st is to use genuine Hella/Phillips/Osram ballasts with the correct HID bulb to fit in place of the oem bulb. This is the better of two options. You will not find these ballasts sold as an aftermarket kit. You would source these from an audi,or benz and pair them up with the bulbs that fit your application.
Option 2 is any of the aftermarket kits, the aftermarket chinese kits have come a long way and if installed soundly will work very well. And are cheap enough to get a spare kit to keep close at hand should it be needed.
Either way when retrofitting to hid, ALWAYS utilize a relay harness to power them from the battery, thus bypassing extended riring throught the dash,etc and much lightening the load on the headlight switch.
Expect to pay $200 to $300 with the good ballasts option 1 or $85 for the chinese ones.
When mounting the ballasts dont use the included mounts as they vibrate and cause premature failure. mount the ballasts with the connectors facing down for gravity promoted drainage of any condensation. Mount them to a CLEAN surface with automotive silicone. When mounted and allowed to dry 24 hours they will be very secure. Please use EXTREME CAUTION when working around energized HID's as the startup voltages can be 35000 volts and will hurt you.
I would also reccomend a set of Hella rallye 4000 non hidlights and then convert them to hid as well. I have on my BOV which is a built 98 tahoe both the high and low beams converted as well as 4 Hella 4000's converted and 2 of the Factory HID Hella ralleyes. the Non HID 4000's are about 95$ per light the HID 4000's were 600$ per light. The difference in light between the factory HID's and the nons that I converted to HID is not much and deffinately not enough to justify the extra $1000.
I have installed a shitload of HIDs and the above is my .02 YMMV