From what I would do, based on my understanding that Source 2’s modeling is synonymous with Blender/3DS, so you should be able to construct it in the same mindset as a model. If thought like that, it should be fairly simple. Unfortunately, I have not compiled into Source 2 yet, so I do not know how well they translate across.
Starting with the size of the model, I would make it 56bu wide and 112bu tall. Unfortunately the picture is too tall for that, so I scale the top of the door up to 136bu. If I wanted to, I could do finishing scaling near the end to shorten it back to 112bu, which should then match the other doors and door frames perfectly.
(Note: I model at 1/10 scale as to not have to deal with a completely massive model. After exporting the model, I add a “$scale 10” command into the .qc file to scale back to intended size.)
What I would do is begin by making constructive edge loops around the model. I would then copy the window face, delete it, and make a new object with only the window face. As long as I keep all the polygons in one object by the time I compile, I am fine, so separating them allows me to clear my head of unnecessary steps involving extra edge loops.
Note: I add a mirror along the thickness of the door so I don’t have to repeat detail on the back side.
I would then add detail by insetting faces where necessary, and finish it off by adding the smaller details such as the door handle.
(Note: I used the Wireframe modifier on the window mesh, with a setting of 3.0 in order to quickly make the window supports.)
(Note: I used a subdivision surface modifier on the metal bits in order to increase the polygons to make them look smoother. Being a model, I could just bake down the details to a low-res version, but for source 2, I’m not too sure how you could go about doing that.)