I had a Navman s/w package running on an IPAQ - its a very good solution and worked well - was a while back though.
The TOMTOM system was also very good. There are loads of these on ebay - "no 5hit" I hear!!!
Not sure how much of an issue it might be, but back then if the car has a heated windscreen, you'll need an external aerial as the wires in the screen will prevent the system from getting a satellite signal.
Maybe worth checking.
Good sites for info -
http://www.pdabuyersguide.com/tips/GPS/ http://www.globalpositioningsystems.co.uk/ Tim
[Edited on 2-1-2005 by Rattler]