I have hid bulbs in my tow vehicle(Yukon xl), and in my daily driver(Chrysler pacifica). Both vehicles came from the factory with halogen headlights. In the Yukon I have a plug and play kit (hid bulbs in original halogen reflector) with a relay harness (uses trucks headlight socket as a trigger for the relay, which has a fused ignition hot only connection to the battery. This removes any extra load from all of the vehicles oem wiring. Normal hid bulbs draw only 35 watts, but spike much higher initially to ignite the bulb. This can sometimes damage the vehicle wiring or other components in the headlight circuit. The relay harness protects the vehicle's wiring and ensures that the hid ballasts have full power from the battery.
With that being said, these plug and play kits are illegal for on road use. A halogen fixture is not designed for the light of an hid bulb. The light is not focused where it should be, and causes a lot of glare for other drivers. Install and use at your own risk(your illegal, non DOT approved headlights may be blamed in case of an accident). Hid bulbs have specially designed fixtures, most of which are projectors. I searched Tampa area craigslist and found the ad you are referring to. That outfit is selling you a plug and play kit.
My Chrysler Pacifica came with oem halogen headlights, but an hid option was available from the factory. I originally used a plug and play hid kit in the halogen reflector headlights, but the output was terrible. The low beam was scattered and not focused where it needed to be, and the high beam lit up the tree tops instead of the road. I took the halogen headlight assemblies out and put in aftermarket oem replacement hid projector assemblies I found on ebay. The output is amazing with a proper projector, but xenon hid lights do have a shortcoming, they do not light up a wet road surface very well.
I think a proper hid projector headlight combined with a good halogen fog and/or driving light is best.
Those hipro bulbs show that they run at 100 watts. Normal halogen headlights run at 55 watt low beam and 65 watt high beam. You may soon be replacing a melted headlight socket, wiring or switch, bcm, and/or headlight housing. I know because I have done it!:S I have had to replace melted sockets, multifunction switch, and have burned the coating off of the reflectors. It was quite dangerous when the multifunction switch burned out because the headlights suddenly quit while driving at 55mph. That was in a ford f150. I ran 80w/100w bulbs in a jeep for years and had no problems at all.
I have purchased all of my hid parts on ebay. I have purchased several plug and play kits from $35 to $80 and have not found the higher priced kit to be any better. I have replaced a couple of bad ballasts (ebay sellers did warranty them). The bulbs sold in this price range sometimes need minor adjustment to fit, and do not expect oem quality or reliability. you may have to tinker with the kit a little more often than oem parts. In the Yukon I carry the original halogen bulbs in the glove box just in case the hid kit fails for whatever reason when away from home.
If your vehicle had a oem xenon hid option available from the factory, you could get all of the parts needed for a direct swap either new or from a recycler. If not, there are guys out there that will retrofit hid projectors into your headlight assembly. It takes quite a few hours labor and will cost a lot more than the $169 pnp seller from craigslist.
There is a whole lot of info on the hidplanet forum about all types of automotive lighting.
http://www.hidplanet.com/forums/forum.php?s=a44da84fa3460bec5efc6b07bc87f546some good info at Daniel stern lighting, too
http://www.danielsternlighting.com/tech/tech.html
