£400ish ought to get you an RNS315 from the current range of cars.
It's got bluetooth, nav, SD card reader and DAB all built in as well as the option of adding an MDI interface. You'll need an later CAN Gateway, a phone mic, GPS antenna and some coding to get it all working.
On the Mk5 you need to remove the trim around the radio and the vent panel on the top of the dash above it. There are loads of how to's on the interweb. If you fit an aftermarket radio, you will need a facia adapter as well.