An alternative to Blaupunkt?
I've just bought a Becker Traffic Pro High Speed of Ebay for £240 for my SL320 and it's fantastic. Becker make the sat nav units for Ferrari, Maserati, Porsche and I believe Mercedes (Mercedes APS 30 is a Becker), so the unit is very good quality, with just a few buttons, very intuitive inputs, yellow display option and it looks like factory fit.
The Traffic Pro High Speed (high memory than other models = faster route calculations) also benefits from twin tuners, so one of the tuners is always scanning for better signals and you have an instant array of stations to choose from. The twin tuners also enable you to listen to a radio station whilst the other tuner locks on to a TMC station, which updates the sat nav and re-routes you around possible delays.
The sat nav side works very well with just a simple input of postcodes if you require and fitting cost me £50. You can now buy new units for under £400 as I think Becker have brought out some newer models, so the High Speed has been kept on as a budget model but it was one of the top models until recently and certainly performs lilke one.
Hope this helps.