I take it you do just mean the radio/cd player, rather than the navigation screen too?
Its a very simple job once you know how. Google and you shall find (I'm not sure I want to present this kind of info on the internet myself, so a PM would be better). If you look for instructions on how to install an iPod kit, such as the Vaistech SLi, you will find out how it is done.
£595 sounds reasonable I suppose, as the only alternative is a dealer and I'm sure it would cost more. Why not call a dealer parts department to get the as new price? If it is not too different, you may get a guarantee as well as knowing that it is brand new and unmarked. Some cars have the Mark Levinson system as opposed to the basic system - which do you have?
You can just change the screen on these units i would look for a screen part number is on the back of the screen. I would also get a price from dealer for a screen as you may find its cheaper than £595. As for getting to the old unit out very easy job just four bolts. I may be too late with this.
Regards
Eddie