Device — any device with GPRS and the ability to install software (in theory).
Some information about car navigation:
1) the Vast majority of devices running Windows CE 5.0 / 6.0 (which is 6.0 — not R2, not R3). And it's NOT Windows Mobile.
2) the Main difference between Windows CE is an embedded system, there is no one distro for all, a system image is developed for each specific device; the manufacturer, at its discretion, configures the system includes or not these or other modules. The image of a specific system to develop software for it and test) can be obtained from the manufacturer (or not — depends on the manufacturer).
3) the Main difference between Windows CE 6.0 with regard to Your question about GPS Intermediate Driver, which allows to abstract from specific iron and a real COM port, and also simplifies the solution of the problem joint work with GPS (Your program and navigation, for example).
More details on this: www.techdays.ru/videos/1162.html
4) Development under Windows CE 5/6 is possible in Visual Studio 2005/2008 (in 2010 no longer support CE). For Windows CE there is .Net Compact Framework (nearly .Net Framework, with some modifications and reductions). VS already contains the emulator for Windows CE. To develop for a specific device, you need either the device itself (connected) or a system image and emulator).
5) something to read. You can start here:
On the basis of this question requires clarification:
1) You are going to develop a program for a specific device or program, which potentially can be installed on several devices?
2) does the simultaneous operation of Your program and the navigation program?
3) About start menu did not quite understand.