I'd be looking for a used RNS-510 on ebay (the factory satnav unit) as they come up in this price range. It has a built in 30gb HDD, SD card support, Satnav, and with extra modules can have bluetooth and iPod connectivity. You may need a special firmaware verison, or a new canbus module to stop battery drain as your car is a 57 plate (borderline on therther you would or not)
Alternatively theres the RNS 315 which is plug and play, has no battery drain issues, has bluetooth and can be had with DAB although it has no built in HDD but it does support SD cards and the ipod modules.
Theres also several Kenwood units designed to fit in place of the stock stereo and they can be had cheaply used too.
Edit: just realised you got one already