there is no wizardry too it, but the subject of HID kits always seem to touch nerves, and i'm not sure why.
because of Alfa's bulb out circuitry, they are very sensitive to changes in the wattage, and because the hid's are fed by a ballast, it is a capacitive load rather than an inductive load, but there are loads of kits out there that are even can-bus friendly, and at no extra money, and these have been perfect for me personally.
chose your colour, 6000k is the typical Merc/Audi white colour, and probably the most effective, but 8000k and 10000k are very popular too if you want a bluey tint.
they are very simple to install, they do just plug in to the standard lamp feed, and come with a loom with integrated relay that hooks up to the battery directly, install the lamp itself just as you do your halogen bulb now (fiddly and awkwardly!) and then find somewhere in the inner wings to mount the digital ballasts, which are getting smaller and smaller all the time so thats a doddle too.
take it along to mot station, check/adjust aim to safety, look forward to your next night drive in safety, and enjoy...
With the 156 and its poor dip beam, aswell as other cars i have them in, and having tried hundreds of bulbs, wattages, and so called claims of gases and colours, these retrofit HID kits, legal or otherwise, have been the only reliable method of gaining more light output.
it has worked for me, and alfared by the sounds of it, but if its not for everyone so be it, i'm only telling of my own experiences.
if its not for you, fine, if you'd rather spend an arm and a leg, and an eternity sourcing the oem units, so be it, thats your choice, as it is mine to spend my money as i see it, wisely, and enjoy the savings and the huge benefit right here and now.
Decent light output needn't be a fortune or rocket science, i just wish i knew what peoples beef is with HID kits?