Windows CE is typically an embedded operating system and is not straight forward to get running on other hardware.
Windows CE is not a simple CD you can install from like the regular editions of Windows you can buy from a shop. It is designed to be a highly configurable*, customisable, very flexible operating system.
It has to be "built" for each target. If you have a MSDN subscription (I forget the level needed), you can download a disk called "Windows Embedded CE 6.0 R3 - DVD (English)" (It is under development tools, I can't quickly see a way to deeplink in). This contains a tool which is called "Platform Builder". I have not run this in many years, but I do not think it has changed.
Using platform builder, you can choose from MANY templates - from a ip phone, set top box, down to full desktop and completely customise many aspects of Windows CE. You will need to target the X86 CPU and most likely will need to find/acquire/build drivers for certain hardware such as the touch screen.
Once built, by default, it installs a trial key which allows the built image to run for a short while (I think 30 or 60 days... but that is off the top of my head). In order to buy a full licence, you need to go to a Windows Embedded Distributor - and for just one unit... you may have a little trouble.
Personally, You may be best off sticking Google Maps (if you have a 3g dongle/internet connection) or Microsoft Autoroute (I think it is) as it has a complete database built in - and is GPS compatible.
As for other aspects, on X86 hardware, it should be quite fast, but do not expect it to be as fast as on dedicated Windows CE devices - many of them are purpose built with flash memory, and BSP (Motherboards that are deeply linked to the OS) for maximum speed.
* You would be surprised how many set top boxes, GPSs and other high end electronic goods run it!
Good luck and it sounds interesting, so please let me know what route you end up going down!