What would I need to get phone bluetooth in my Golf?

[TurboHatchback]

I have a 56 plate GTI with a multi-function steering wheel but the basic CD player. What hardware would allow bluetooth phone connection using the steering wheel controls? There seems to be lots of different units (RNS-315, RNS-510, RCD-510 etc etc) which all have a phone button on them but I'm not too clear on the differences between them. Do they all support bluetooth & is there more to installing them than just plugging in the unit and everything working perfectly?

[J400uk]

315 is the only one with Bluetooth built in but these don't support the Highline screen in a Mk5 which limits there usefulness. Others require a separate Bluetooth module which costs anything from £50 for a Chinese import to £150 upwards for a UK item