Dave is right ... only a full OEM retrofit will have the autoleveling ... and will be very expensive (prob. thousands of pound) ...
As a guideline...
Typically all the HID kits are priced similar and can be fitted easily. You only need to check what bulb you currently are using.
Installation is normally straight forward. Take out your existing halogen bulbs, put the HID ones in, plug the HID ballasts into the original power cables and plug the HID bulbs cable into the ballast. Then try to find a place to secure the balast.
Most kits like the H1, H4 low-beam only), H7, 9006, etc kits are priced similarly. Price may vary depending on the color of the light... e.g. 4500K, 6000K, 8000K, 10000K. The higher the K the more expensive the kit.
In practice, everything over 6000- 6500K is getting blue-ish to purple, and is not really usuable. It is more for show
In addition, some H4 users require H4 high and low beam HID kits. These kits are much more expensive, since the bulbs need to be capable of switching between Hi and low.