There are quite a few 'gotchas' when upgrading the various components of the Nav system.
The simplest upgrade is the Nav computer itself, this is pretty easy and there is not a lot to go wrong. If you have a Mk II then you have to bypass the trimble (do a search), otherwise it's easy plug and play.
Next up is the 4:3 -> 16:9 monitor. One gotcha is that the older TV modules will only output 4:3 even on the 16:9 screen, you just get blank bars down the side. So you would have to either upgrade the TV module or remove it.
Why upgrade the TV module when analogue has been switched off around Bath, might as well just take it out, no? Well, there are two possible uses for an Analogue TV module. First you may want to watch DVDs, it has an AV input, although only with Mono audio. Second, it has a backup camera input. You can also upgrade to an OEM Free-view tuner, but it's very expensive.
Bypassing the TV module is usually quite easy, just unplug the TV module and replace the blue plug in the Nav computer with the blue plug that was in the TV, with some cars this will stop the Nav audio instructions from working without a few wires being moved, in most it will be fine.
You're car probably has the Traffic Master system, my advice is remove it before it fails.
DSP can also cause additional complications when installing an iPod interface. It uses a digital CD Changer audio input, all iPod interfaces that have decent audio replace the CD Changer source so you need an Analogue to digital converter to get the signal into the DSP amp.
Pretty much anything that is not plug and play, or very nearly plug and play, will mean that you will have to strip out the entire system and start from scratch, there is interaction between the radio and amp, remove one and other stops working. The monitor is 'dumb' it will only work with the OEM radio (or a specialised car PC). On top of this, most of the connections that you'd expect to be behind the dash are in the boot, speaker connections and antenna etc.
I'm down in the west country, one stop down the M4 from Bath regularly, if you want to have a chat or demo of what I have, let me know, next time is Monday 10th Jan.