my zed came with factory halogens, can i purchase a set of factory xenons and fit these or is there as usual, much more to it than simply swapping them..
There is certainly a lot more than just a retrofit (although possible and expensive). You will need the headlamps, self levelling devices, wiring looms, interior switching, interior cabin switches etc etc. Far better to replace the OEM bulbs with aftermarket lamps e.g. Phillips X Treme Vision but there are many others.
Exactly what Scooba steve says. If you go for a factory kit then its expensive and a pain in the bum as you have to install a load of bits and pieces.
Go for an aftermarket kit and its about £50 - £100, doesnt require a controller and easy enough to install. I paid £90 for mine installed. just make sure you get the correct kit for the car. The balasts in the aftermarket kits are different and the Z4 ones ensure that you dont get bulb warning lights on dashboard when lights are functioning correctly.
If you are london based, for what its worth I used these guys and they did the install for me while I waited. I was in and out of the garage within an hour.