Mini flight simulator over static 3D terrain

My task is to write a mini-flight simulator, one that has a fixed point of view from the cockpit of any plane, without even seeing the cockpit or any other part of the plane.

The terrain above which the aircraft will fly should be taken from an existing GIS repository, for example Goggle Earth or another source if any. The flight area can be a of limited area, for example several hundred square kilometers of desert with roads and streams, so there is no need to connect to an infinite real database, or in other words, the map can be static.

It is desirable that the 3D terrain be as realistic as possible but there is no need for a very high level of details. The flight above the surface will usually be at an altitude of about 1,000-2,000 meters and not lower than a few hundred meters, so that there will never be a touch down or landing.

I suffer from inexperience in graphics programming except for basic things but by no means in 3D, so my questions are basic and they are all around the question of where to start.

I would appreciate any tip and would love to expand where it is not clear what I meant.

Thanks in advance.