2000 E55 Radio
There is a Metra adapter harness available which makes it plug-n-play - with a Euro antenna adapter for the radio antenna lead to your new radio - you are best served with a new GPS antenna that looks like a "hockey puck" that can be mounted up inside the dash with no real problems
Your headunit with Nav is termed "double-din size" - I am certain you can google and fin some install video or directions somewhere - or any competent shop can do.
Yes - you loose your steering wheel system controls .
The key here is 3V or 3V+ pre-amp output - and w/connecting harness adapter this is almost plug-n-play



