Please don't buy the total out dated Avin Avant 2, it runs on the outdated Jelly Bean and a dual core cpu, if you want to read more, google E46 fanatics. It isn't worth the 800 Euro
I can really recomend you the Ownice C200, it's a quadcore with 2 gb mem,
Ownice also delivers For Navall, they call the C200, the Navall 3, And they have a install/repair shop in the UK
If you can wait and want an even faster cpu, this month Ownice releases the C500 with an octacore cpu with 4 gb mem. in a expo in Moscow, Russia, and in September a messe in Frankfurt, Germany, so you got to wait until september/october.
Otherwise... If it's pure for the sound... Alpine released this expensive car radio special for the E46.
http://www.alpine-electronics.nl/p/Products/navigation-system-for-bmw-3-e46/ine-w997e46