Some of what appeals to you in the source image belongs in the painting simply because you like it. If something connects you emotionally to the scene, it must be important. You may still want to change it in various ways, but, by all means trust your feelings.
Most of what you see, though, is not so easy to place on the scale of what is essential and what is optional. This is where it helps tremendously to make sure the painting progresses from the general to the specific.
Generally speaking, there are only a few major shapes in this image: sky, street, 2 cars, big triangular group of buildings on the right, strip of middle value buildings in the background, and the pale, cool skyline in the far distance. In order to understand where these shapes are in space, I need to make them different enough from each other to stay separate, but similar enough to feel like part of the same scene.
At the early stages of the painting it is not necessary to look more closely, or more specifically at the shapes to begin planning how to meet those general requirements. How would you separate the skyline from the strip of low buildings, for example? You are not trying to do any more than locate the shapes at this point, which can be done without including detail. The skyline could be a flat shape, in a single color, with no texture at all. The nearer buildings would then need only a little color variation and a second layer suggestion of windows and doors, or a slightly wider value range to meet the immediate need.
What about the dark wedge of buildings on the right? Once some of the other shapes have been described you have a basis for deciding how much of what you see in the next area will be needed. Start by identifying the most general aspect of the shape (big, dark, triangular). Would that be enough to get it to do its job? If you are unsure, try it as simple as it can be. If it feels like it needs more information, look for the next most general aspect. Since these buildings are closer, you may expect to see more specific information. In that case, it would help to at least suggest that the shape is made up of several smaller shapes. What would make this simple triangle look more like a collection of individual buildings, without calling too much attention to it? In this photo, there isn't much to draw from. Instead of peering more closely, in the hope of seeing something there that will provide an answer, you can look to your own sense of what makes a building a building. How about vertical strokes, diminishing in size as they step back in space?
The movement from very general to increasingly specific can be made in increments. Thinking minimally like this allows you to make decisions about what does not need to be included. If the job you have set for yourself has clear parameters, it will be easy to know when it is done. Standing back from the picture, then, you can tell whether you want to add another layer of information.