Hi
I have had a conversion kit fitted to my car for the past 4 years or so.
Technically they are not legal but you can get by if you have a MK1 with projector lamps, your headlamp leveller works and your headlamp washers work.
I have 4500K as they are close to what the OEM standard was for the era so they do not attract attention.
As for the brightness compared to the standard Xenon bulbs there is no comparison as the standard factory set up for the quality of the car is pants.
I would get a kit as the prices have come down and they are so easy to fit being 'plug and play' and if you ever want to change back you can, but you wont!