Aluminum door and side window help

We have recently moved into a house where it has a 1980's Everest aluminium porch with a glass door. We would like to put in a cat flap and would ideally like, to either replace the door with one where we can cut a hole in a panel(wooden or upvc?) or take the side window out and infill this with something. Not sure where to start or which would be the easiest & least costly.

 

I can see I've posted this in the wrong place Please ignore me, I'm new!