Happens I'm the person to ask, since I added an RNS510 to my Eos
Since your Eos is from 2008, you may need to replace the gateway with a newer version if you have an issue with battery drain after installing the RNS510.
You will definitely need two things: a GPS antenna (since you have an RCD500 the car won't have one. You can get one and mount it under the dash above the radio) and a dual Fakra adapter, which looks like this:
The RNS510 has many configuration options ("coding") - if your car has the Dynaudio sound system (amplifier under the left front seat and Dynaudio emblems on the door cards), or if the car it came from had a sound system and yours doesn't, it will need coding. You can also change the colour scheme, enable the test menu, and other stuff.
To update the gateway and to configure the RNS510 you will need a diagnostic tool like VCDS or OBDeleven - see the link in my signature. You don't need a dealer to do this.