Thanks for info so far. So far I have on the way a MK4 unit, GPS antenna & a frame to mount the unit into. BMW Oz seem to be the most accessible source for the rest of what I need at about $500-$600.
I would really prefer not to run any cabling the length of the car but if I have to then so be it. BMW say that they sell a small loom that attaches to the main loom in the rear and that is all I should need, we will see. After the main bits arrive and I have fitted them I will roll into BMW Sydney and see what I am missing and buy the bits from them.
I have seen a few retrofit guides on this and other sites but they all seem to involve fitting the full loom and monitor, etc. I hope that whatever BMW Sydney have is simply plug in or with adequate instructions.
The next thing I will need is a CD map, BMW Oz want $330 for it which is not too unreasonable I guess but I would prefer to buy it on ebay or somewhere else for less!
I have seen a few maps on eBay but they seem to be mainly V12 do not seem to be for E53 X5. Any clues here would be greatly appreciated!
Again, thanks for all the replies.