Repairing site of door in exterior wall

When the door was permanently closed in a brick exterior wall last year in a home I am buying, I'm guessing the interior aperture was covered with sheetrock,taped off and repainted.Now the tape at one corner is coming loose, and an entie vertical edge is cracked.I want to keep the door appearance from the outside.What should be done? Generously caulk the door edges?Create a series of horizontal and vertical studs?Insulate? How big a piece of sheetrock-door size,panel size or entire wall? Hank

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