I may be mistaken but as far as I know the "BMW Assist" and the "Bluetooth" are controlled via a single unit and that has nothing to do with the navi system.

You may already have the GPS antena so you can save some $ on the GPS antena and the antena cable but you do need to get the 16:9 and the MKIV and the retrofit cable and the misc parts from Vaheh or elsewhere.

I have recently done the retrofit on 03 4.4i but if you have 3.0i, you might want to do the High OBC(SA555) upgrade at the same time in order to get the Time and the other High OBC functions on the Navi screen. This SA555 upgrade will be another $450-$650 on top of the Navi upgrade. If you decide to do SA555 upgrade, go with the 4.6i instrument cluster - it looks much better than 4.4i(I am not so sure if you can use 4.8i's cluster since 4.8i's cluster has 6 speed manual indicators instead of 5 speed on 4.4i and 4.6i)

Good Luck~