Don't know if I fully understood all issues right.

My imagination would be that:
- you set the desitination manually
- get the actual position from GPS
- calculate the best route from here to destination (which is a rather and probably the most complicated task)
- transmit this route to gpsapp (prob. as shapefile)
- best case but not essential: gpsapp draws the map around the actual position (but arm could be overloaded)
- follow the route with gpsapp
- if you leave the route gpsapp gives the user a notification
- calculate a new route with the actual GPS position as start point (manually or automatically triggered)
- restart gpsapp with the new route

So the critical tasks would be to read map disc data, calculate routes and generate a input file for gpsapp.

This is in fact not as comfortable as a commercial navigation system but at least suitable for frequent drivers.

Juergen