E53 is the entire X5 model range - whether the original model (99-03) or the refreshed (04-06).

You can certainly upgrade to xenons. But it will be a pricey upgrade. Someone who has done it can break down the processes and all the parts you'll need. I certainly do know that it isn't a easy simple switch the headlight out DIY (different wires, ballasts, etc...).

To upgrade to the new 04' light look is going to be a little more difficult. In order to get the new lights, you'll have to switch out several parts which include, the front bumper, the lights and the hood. Its a beautiful upgrade and I would highly recommend it if you have the $$. Also remember that you can save a bit by upgrading your car to the 04' refreshed look and just buy the new refreshed lamps with halogen's instead of xenons - that will save you quite a bit of $$.