"Is it ever appropriate to hire an ADHD coach for a teenager?"
Many coaches do work with teens, but not every coach will come to your home and help them go through closets and set up study areas. If your daughter needs "hands-on" assistance, consider hiring a professional organizer (you'll find a nationwide directory at Napo.net and more in the ADDitude Directory). Make sure the organizer you hire is patient and non-judgmental—ideally, this would be someone who specializes in ADD.
Once your daughter's organizational system is in place, it might help to have a coach check in with her once a week to make sure she is using the system effectively.