I don't have my LR3 yet (next friday hopefully!) but I'm a Discovery II owner and have driven / worked on LR3s before.
Sounds like an impulse buy to me! Especially if you were looking at a pickup truck, but I'm glad someone seems to have convinced you to buy one!
From what I had read, the headlights (and inclinometers for the self-leveling headlights) are enclosed within the headlights. Perhaps it's a fairly simple reprogram of the car's computer system; shouldn't be too difficult, especially if you know a LR specialist that can reprogram modules. The headlights are going to be quite pricey however! Probably ~$400-500 per side.
The nav screen is much the same deal, everything in these vehicles is fiberoptic, nothing is analog like most vehicles out there. I'm not sure if the "wiring" harnesses are run to the nav location or not, HOWEVER the nav screen really has nothing to do with the stereo. Even vehicles equipped with nav and the Logic 7 system have fully independent GPS systems. They show nav and suspension information only (and some diagnostic info if you know the secret codes

)
There are aftermarket ones that fit, but it's not quite the same thing - and like I said, the stereo on the LR3 was never controlled by the touch-screen nav system, which was only for navigation and suspension purposes only.