If you want to stick OEM and get the RNS which has nav etc then this is why you'll need:
RNS510 £550+ (for rev c with led screen)
MDI kit ~£120 (depending if you have it in the arm rest or glovebox)
Bluetooth kit ~£200
Depending on the age of your car you might have to upgrade the can gateway to avoid battery drain which is roughly £80.
You can get the above cheaper but they are rough prices. Also depending where you are there are people who can get hold of all the stuff you need