No no no the original stuff is all going. I don't see how that would work Daz as it's not like the OE screen has a 'video input'. You can buy devices to input video to the screen but then that would only be useful if you want to keep the OE system, which I don't, and doesnt work for audio. As you found you had to buy a seperate head unit anyway.
You see the Sat-Nav is good, but way out of date, the audio is poor quality and featureless (CD (no RW or MP3) and Radio only), the climate control is a gimick, and overall it's a very inefficient use of space (two drives taking up two DIN slots, and controls taking up 2 DIN space, with screen uptop 2 DIN. The controls arent even that well designed). Again, your sperate head unit had to go in the glove box as there was no other space. So yeah, OE stuff is going.