2003 model

what year? if its the latest body style (06 & up) it will be hard, almost impossible. But if you have the model before that, then you can pick any in din-size stereo unit, get an installation kit and wiring harness + antenna adaptor and it will all fit and work nicely.

There are probably lots of headunits that will match your BMW. You will also need the PAC OEM-2 Universal Factory Sound System Interface to use one. These can be found at http://www.crutchfield.com/S-hcBE6w0D59E/cgi-bin/prodview.asp?i=127OEM2.
You can also enter your car year and model into their website and it will tell you all the head units that fit this particular car.