You can change your screen, change your nav computer, or change both. All of these combinations will work. TV is optional. If you already have TV it will work, although maybe only in 4:3 aspect ratio.

Taken your present configuration the 16:9 screen head unit is plug and play. You just remove your current unit and insert the new one in its place. Connections are exactly the same.

Same goes for the navigation computer. Just exchange your MK-III for a Mk-IV. Same conections, same rack. Should work right away.

You may want to re-code your new nav computer to your car VIN. It is not necessary but it is a neat thing to do. You can do it yourself if you've got a laptop and a iBus to Serial interface.

Good luck