When buying a Bluetooth module, make sure its a High model as the low model wont support it.
I purchased a complete used kit, the module is 8P0862335H got mine from vag_retrofits of ebay, expect to pay about £150 for a kit.
do a Google search on Audi A4 B7 Bluetooth and read all you can.
As for RNSE, I bought my car with it factory fitted, it works fine and update maps but its no as good as my Gamin satnav.
I would also consider an Android unit instead, and you can connect your phone to it to make voice calls.
Do your research before you buy.