Why not get a Tom Tom Navigator?
In the past I've owned a GS430 and two LS430s. All had sat nav.
At the moment I have a Subara Legacy GT Spec B. I need an estate and didn't fancy the IS Sportcross. There is no sat nav in the car so I got the Tom Tom Navigator.
I think this is actually better than the Lexus sat nav system. Benefits include:
1) You can use it in more than one car
2) You can upload speed camera databases to it here:
http://www.pocketgps.co.uk/uksafetycameras.php
3) The software is much better, does postcodes for starters. And the voice directions are better.
It costs around £400.
Batteries last for about 5 hours, but you can power it and re-charge it from the cig lighter socket if you want.