You COULD just go with a cheap eBay "HID upgrade kit" that will include a new HID bulb and ballast that is meant to go in the same spot a regular bulb is...but those are illegal and dangerous. If you care about others on the road, you'll not go that route and instead choose to add a proper HID assembly to the existing headlight housing. A little work, sure, but it's legal, unique, and responsible.
FWIW, usually, you waste money on "conversion kits". All the good bulbs and ballasts come from Germany (Osram and Hella), while nearly all "conversion kits" come from China. The bulbs and ballasts are cheap, will probably need replacing within a year, and don't provide the best light. There is more to quality headlamps then how bright the beam is. Heck, you could have a true 120W (very bright) bulb installed, but if it were red, it's useless as a headlamp.