I researched this when I did it on my Signum.
Get a decent kit not a 20 quid ebay special. Get bulbs with metal holders and base. If you do it right HIDS can make a massive difference to the cars looks and to how much you can see at night.
First off I think they only look good on light clusters designed to have them, where a higher model of the same car might come with them as standard. - e.g. projector lenses. On none-projectors they always seem to just dazzle.
The law is hazy and it is down to the individual MOT tester or policeman IMO. The law for HIDs basically states that headlight cleaning and autolevelling systems where fitted must be working. SO if you have OEM HIDS as standard your cleaning/levelling systems must be working.
For aftermarket HIDS some read that as if they aren't there in the first place then they can't be working or none-working so it doesn't matter, others read it as if you fit aftermarket HIDS you should also fit cleaning/levelling systems.
I know people who have had aftermarket for years and sailed through MOTs, others have come a cropper.
I would advise if you are going to fit them try to get a colour and Kelvin rating as near to OEM as you can. Don't have incredibly bright or weirdly coloured ones and you won't draw the wrong kind attention and definitely make sure they are aligned correctly.
One important note too is that they run cool, and if you put them in clusters in place of normal blubs you can get condensation. I used sachets of silicon gel inside the cluster to stop this.
To avoid it altogether site the ballast inside the cluster and make a proper seal where the wiring comes out.
Research your car model, on some cars, as HIDS run low voltage you need a system with a relay to stop the computer reporting a failed bulb.