Lovely cars, i loved mine, did around 100k miles in it over 3 years so very heavily used from me, and burnt a whole lot of fuel in the process
Re the bluetooth, you can get a bluetooth puck which will enable the phone system, it sits in the existing cradle and the car sees it as a compatible phone, it will look loads better than that aftermarket kit (which i think looks terrible).
Re the comand, you can update the system to a D2B 16:9 unit from a MY2003 if you wish, this was a crossover year where the cars got the widescreen unit but still kept the old D2B bus. 2004 onwards the system totally changed. The unit from a 2003 is PnP with your car.
Other upgrades, i would stick some better wheels on it (mine had 19" AMG rims, and looked very good for it in my opinion), better wheels take years off them
Another big big one, which mine had, was 2003- lights all round, they look loads loads better. The headlights and rear lights really lift the car and bring it inline with the W211 E-Class really, which was produced all the way to 2009. Also worth doing are the wingmirror repeaters which can be swapped with 2003- clear LED items.
This picture is years and years old now at an old OcUK rolling road (i dread to think what the car looks like now
) but it looked very smart in its day: