Radio/dvd/gps system

[disco gazza]

I only have the radio/cd in my HSE and was wondering what i needed to buy if I wanted to upgrade to a radio/dvd/gps system.
I have seen the unit itself is between 450.00 and 750.00(fleabay).
But what do I need to make it all go if I dont have the bits in the rear left part.
Pic is off what I've got in there at present,which isnt much. ;)

Thanks

cheers