I guess the first thing is where are you based, as a local forum member might install it for a beer. As you might need some coding, so VCDS cable to interface with the car.
The can gateway can be in one of two places, on older cars they are behind the ashtray on the center tunnel for the exhaust, on older cars they are up by the peddle box, they are a small module that controls the information transfer between all the computers in the car.
The issue can be that the older modules dont have the right coding to tell the third generation stereos to power down and will continue to draw power leaving you with a flat battery. So the unit will work and if you use the car regularly enough you might be alright but if you do have issues the replacement can gateway can be bought for £70, this you will need to re-code but a local forum member should be able to help you out.
The only advantage with an RNS over and RCD is that they were introduced prior to the change in the Can gateways so with certain firmware they can run without battery drain issues, so it may actually be cheaper to buy an RNS in the short term rather than an RCD plus gateway.
Especially as it will be much easier to fit the RNS than RCD and gateway.
If your near Bromley or somewhere in the Surrey area I would be happy to help you out.
