Design departments rely on a lot of written communication. You can tell really quickly in a meeting if it's going to work. You need someone who has a feeling for story and emotion, rather than architects, who are better with space and moldings. The shape, flow, and pattern of the acting space color a movie. Don't be afraid of hiring theater people for costume design.
Commercial stylists are very different than costume designers.
They don't know how to cut corners or define characters by the clothing they wear. If there's a designer whose work you really like and you know you don't have the money to afford her, don't be afraid to call anyway. You have nothing to lose. They may fall in love with the script, or more realistically, leads you to others who have a similar style whom are willing to work for lower fees. Make sure your accounting department is functioning before your designers start.