Those prefab coops are typically not really designed for chickens, don't hold nearly as many as the advertisements say they will, and can be poorly constructed with poor materials. Before anybody buys one I think it is a good idea to post a link and let us critique it. You can occasionally get a decent one but usually they are not worth the money. You are generally better off building one or converting a building like a shed. But if can also be a good idea to let us critique what you plan to do before you start building. We may be able to help.
If we knew what you are working with it could help a lot with suggestions. Photos would be great. Is it human sized or chicken sized? Ground level or elevated? Will you be opening and closing it from inside the coop or from the run? I've built a human sized door with hinges on the side, a chicken sized door with hinges on top, a guillotine type chicken sized door that slides up, and a door that slides to the side. I haven't done it but I've seen a small door that hinged on the bottom so it forms a ramp when it folds down. All kinds of options. Knowing what you are working with could really help narrow down suggestions.