You can make a list of things that might be ideal (an expert, an experienced academic) or preferable (someone who is nice, someone who could be a good reference), but they’ll never be perfect.
Steer their selection in conversation with your supervisor. Find out what you need to know about their work. Get ready to talk with some experienced academics about your PhD work.
They’ll be good, maybe great, but don’t put them on too high a pedestal.