Tear out the door frame and frame in the lower half if keeping the header height the same. If header height is changing the entire opening needs to be reframed/reworked.

+1. You need someone that can handle the demo and the put back. What does the exterior that is on either side of the door consist of?

Outside is vinyl siding

Make sure it's not against code to remove door.

Good info above.

1) Make sure that entrance is not needed per code

2) Decide the size of window, as long as you dont make it wider the cost can be minimized as a new header and everything that comes with that won't be needed

3) The existing unit gets pulled entirely, a new RO is created and window installed. Insulate, drywall (or whatever your interior finish is), and button up exterior with sheathing matching siding.