As previously said, the only real modification needed is a hole in the rear light cap for the wires.
For a good quality job you'll also want to screw the mounting bracket for the ballast somewhere solid. I stupidly used the supplied Velcro on the first set I did. This was a mistake. It came loose after a while and the ballast ended up swinging in the wind.
As to kits. Buy the best canbus supporting model possible. They are better built and all in one. The non-canbus models have a transformer in the power supply leads which is a pain to secure without putting strain on connectors and the electronics aren't as well controlled for voltage use. Interweb rumour suggests the non-canbus versions don't last as long.
On colour, go for 4300k-6000k. Any higher gets blue and is like an invitation to the f...., er, the police.
When installed, go for a beam alignment check and ask if the pattern would pass an MoT.
The MoT rules were due to be changed to outlaw aftermarket HID kits, but the spineless legislators backed out at the last minute. That said , insurance companies, known for their wriggling /slimy natures, could say an Aftermarket HID kit isn't insurable as halogen headlamp units are tested and E marked for halogen bulbs, fitting a HID kit breaks the compliance with manufacture requirements and could be used as a reason to not pay a claim, or to blame you for an accident if the other party uses headlight glare as a mitigating circumstances for the crash.
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