Yes you can install it into a E320. There are some companies that make a plastic insert that will allow for the Double din install - I have seen them online. If you have the bose amp you can easily wire the system up through the stock harness by using the VW wire harness (crutchfield or any other source) you will also need the euro converter for the antenna.
If you have the bose amp , you need to purchase a line level converter like (Scosche Oea4 4-Channel Adjustable Factory Amplifier Interface ) becasue you dont want to fry the bose amp by feeding it too much power. You can also gets l