Functionality of the module with wiring harness is 100% alike the functionality of the module mounted in the boot. Meaning, MFSW works fine for skipping tracks and changing volume, sat. nav continues to work (if you reattach the C-plug properly :wink: ), etc. However, the telephone functionality (taking calls, etc.) will not be working for taking calls via the module , as the module is only wired into the CD changer connections. You thus simply have to do that on your phone (but if you do not have OEM telephone functionality, that will not be different from your current situation and - other than now - you will be able to continue any call you started or accepted on your phone handsfree, without seeking your headset, etc.)
Furthermore, as already mentioned above, rumor has it that in terms of DSP compatibility, the one directly behind the headunit is better than the one in the boot (if your German allows you, you could do some background reading on this on the German forum).
As for the CD changer: you will loose that functionality for 100%, as the module
pretends to be the CD changer. That is exactly the same for all four varieties if this module. These modules use the CD-changer connections (whether at the headunit, or in the boot), not the AUX connections as some cheaper alternatives (with poorer sound quality) do.
This module really has an extreme rich sound quality compared to AUX alternatives and comes at a fraction of the costs of alternatives with more functionality (such as also displaying tracks in your screen). IMHO thus more than worth giving up CD-changer functionality (I have to encounter the first CD not being available via Spotify and the likes. Moreover, this way you can also listen to whatever is available on your phone, such as internet radio, podcasts, etc.)
I have no financial interest in saying so, I am just massively impressed by the value-to-money these modules offer.
As for the buttons; yes, these are changed to X3 buttons, sourced from the local dealership. By the way, if that would have been the only changes made to the interior, my wife would have been a whole lot happier (You are looking at a very early '03 model. By now roughly 60% of the interior has been changed against OEM FL parts, and there is more to follow

)