Going to an RCD310 or RCD510 will be a huge improvement on sound.
as for the Aux input for the RCD300.
Part number 5Z0 051 434 A
Just going from a compatibility list I have which is stating compatible with Touch Phone-Kit
I personally went RCD310 and MDI, never looked back. Now have a charger, full access to playlists etc and shows my song information on the radio.