xenon headlights equals:
*bi-xenon (shutter) projector lenses
*self-levelling sensors connected to suspension arms
yes, you could buy the full headlight assemblies, but cost is likely to be extreme. i know that the simpler design GT genuine xenon assemblies are around AUS$3000 each side, though that includes the ballasts. probably a bit less in the UK.
i don't know if the non-xenon Brera/159/Spider is completely different, but with xenons, high beam is via the same xenon lens (hence 'bi-xenon' meaning an electric shutter mechanism that lifts up for full beam), and 'flash' is via the 3rd lenses (the middle-most lenses). in your car, is high beam also via the same lense/bulb as the low beam (ie: dual filament in the case of halogen blubs)?
it means that if you fit a conversion kit (HID globe + ballast) into your existing lenses, you will not have high beam, so that equals FAIL. cos it's not dual filament (and i don't think there's such a thing as HID dual filament).
so whilst it's do-able to fit genuine assemblies (but without the washers and self-levelling which in itself could pose a MOT issue), it would be very expensive.