Which mobile platform should be used?

[gurge60]

I only have some experience developing for Windows Mobile based devices, so this is only a partial response to your numerous questions (I am also interested in hearing some discussion on this topic):

Here is some documentation from Microsoft regarding GPS app development:
http://msdn.microsoft.com/en-us/library/bb158727.aspx

The Windows Mobile 5 and 6 SDKs contain samples that show how to work with GPS data in both C++ and C#. I don't know if GPS development is possible with pre WM 5.