Why not just use the screen from one of the mp4 players? It is a fairly simple task to write a display driver for it on a PIC or other cheap microcontroller. For example, the nano clones which you can pick up for about £15 use a PH128128 1.5" CSTN screen, driven by an SSD1779 driver IC. They only have an 8 bit data bus, a couple of other pins and a backlight input.
It already has in built functions like draw shapes, lines etc. All you need is a microcontroller, memory IC loaded with your map images, you can take map images from google earth or somewhere without having to be too complex and use existing GPS software, and correlate it to your GPS signal. If you need communications you can use the UART on the microcontroller.
You really wont get an OS to run on the chip provided with the player(ATJ2085 on mine), it is a dedicated audio player IC with very little capacity to run anything else. If you trawl through the datasheets, you may get your own software running on it. I wouldn't know where to start as information on it is very limited, I can't even find an instruction set summary.
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