You've got 4 choices.
Cheapest is the 'extra bright' regular bulbs by Ring or Osram. They offer 120% more light than a basic halogen bulb and appear whiter/blue tinted.
Next is a HID kit in the stock headlights. About £35 gets a canbus compatible kit. Lots of light, but in reflector lamps also lots of glare. They will generally pass an mot but lots of people point to new mot rules and all that clap trap.
Next is a set of OEM HID lights. Buy a set of Alfa made HID lamps and swap them on to your car. Expect £250 for the lamps, and a lot of labour if you want all the self levelling to work correctly.
Final option is converting your existing headlight units to 'proper' HID using projector assemblies grafted into the stock Alfa lamps you have. The kits start at about £120 and then lots of labour to split the lamps, fit the projectors and re-glue the lamps together.
It all depends on what you want. In order of light out put it goes Upgrade bulbs, Hid kit, projector hid grafted in, OEM conversion.
In terms of price, it's the same order.
What do you wanna spend?