As pvr says, if you have the nav screen there is no point getting anything other than the Intravee. It's actually going to cost you about £200 as you'll also need the Alpine iPod interface.
Basically the Intravee converts the BMW ibus data into Alpine AInet data so you can then connect AInet compatible devices (in this case the KCA-420i iPod interface) and your car will see this as a CD changer. The Intravee acts as the controller, taking data from the iPod and formatting the text so it can be displayed within the limitations of the BMW CD changer display onscreen.
It's true that you cannot have this connected at the same time as your BMW CD changer, but there are options available if you want both. Because the data-bus is converted to IAnet, you can connect other AInet devices. So, you can have the Intravee connected to an AInet hub, which has the iPod interface AND an Alpine CD changer hanging off it. It's hard to describe in writing, so have a look at the diagrams here:
http://www.intravention.co.uk/forum/index.php?topic=1109.0
Juicy, I'll get the part number after I've had my lunch... I'm starving.
Spandex