i have heard fitting HiD's are a bit of a grey area .. so was trying to avoid getting swept along with that debate ..
Strictly speaking, to retro fit HID lighting to vehicles with OE filament bulb lighting, the vehicle must meet 3 criteria:
1. Have self levelling headlamps.
2. Have headlamp washers.
2. Pass a MOT beam alignment inspection test
http://www.nidirect.gov.uk/customer-information-notice-cars-2010.pdf
I have partially upgraded the headlamps on my 80 to HID and it passed the MOT without issue despite having manual headlamp adjustment. As far as actual lighting performance goes the fitting of HID's to filament bulb housings seems to be a 'suck it and see' situation with some performing much better than others. After experimenting with a couple of HID kits here's what I found:
H4 HID's in the outboard high/low beam housings gave rubbish results on low due to beam scatter. The light was all over the place and adjustment made no difference. High beam was pretty good though.
HID's in the inboard main beam headlamps (H7's I think) gave very good results with a good pattern and effective adjustability.
I also fitted HID's in the a pair of Hella Luminators I have on the front bumper and these were excellent.
So the setup I went for was to leave the OE halogen filament H4 bulbs in the outboard headlamps to avoid the problem of the poor beam pattern dazzling other road users and fit HID's to the inboard mains and to the Hella's.
The difference in lighting performance is MASSIVE!. The only 'problem' I've found is that when I'm on full mains and I dip for OC traffic it's almost like turning the lights off altogether, the difference is that great. I already had upgraded halogen H4's for low beam and they appear a dull yellow compared to the HID's.
The brightest or whiteist HID's are in the 4-5000k range. After that you start to get a colour tint with a gradual fell off in actual lighting power. I went for 4300k.