Keep in mind if you redo the drywall that in itself is a messy job as well with mud, taping, and sanding. Even if they just drywall over the old drywall. It's less work and easier but not less overall time. I'd also ask about anything that comes through the area like doors, windows, outlets, light switches, heating and air vents, etc. Potentially all that needs to be reworked too if they just drywall over and don't remove the old drywall. In our case we did the removal (took a day), painter did a skim coat and painted it a few hours later.