I have a 04 X5 with assist no NAV...I would advise you to get the 05 Bluetooth/Assist module from BMW, they cost about $1,000 and you can work them for a better price. Do this and you will save a lot of headaches...Who is your provider..??
Answers to your questions.
1. forget cheap will cost more in the long run.
2. yes the present assist module goes, dealer will remove when they install new Bluetooth/Assist module.
3.No the voice module is another cost to add.
4. Yes your car needs to be recoded by dealer...
Note: Most but not all 04's can get the 05 module, to see if you can call your dealer with the VIN number...

Good Luck,

X5Strap