Exterior door install question

I'm replacing an exterior door, with a new prehung door. So I'm removing the old frame. The problem (I see now) that there is a window right next to the door (the kind that is a foot wide and as tall as the door.) When I took out the old door trim, I realize the door frame is not that stable. The brick molding seems to actually have stabilized the door frame, as the molding was nailed into the house frame. Is this sort of door supposed to bought as a unit with the window, or can the window be left in place and only the door replaced? I also wonder how I will nail in the new door frame. There isn't much solid to nail into on the side that has the window - no door framing stud, just a strip of trim that separates the door frame from the window. So the new door frame would only really be strongly supported on 3 sides. Or am I missing something?
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Sounds like you have a door with a sidelight. Theses are purchased as a unit. Some are "box" construction with the sidelight/jambs and door/jambs fastened together and it is possible (but not worth the effort) to remove just the door. Some are built as a single unit (better IMHO) , i.e. continuous sill, and cannot be separated.

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