Sounds strange to replace a door for a dent, it could be the door has extensive damage and can’t be repaired but they don’t want you to know as you might refuse the car, I would ask for pictures of the damage before I even considered accepting the car as it could have all sorts of problems down the line, don’t let them fob you off as they would have had to take pictures to get the repair authorised and if they say you can’t look at them or haven’t got any then I would be asking for a report on the exact damage that has been done as you could end up with a lemon for 3 years.