Keep the bird in whatever it is used to travelling in. Put fruit in the cage if you can--- apples, pears, carrots etc have lots of water in them (find out what fruit/veg the bird eats). Keep bird away from draft of AC etc. I wouldn't let the bird out in the car because it is going to be very stressed as it is and that could just make it worse--- plus, they aren't very stable without a perch.
You will want to put the bird in the back seat next to a person (if possible---just because airbags are dangerous and someone needs to be able to watch the bird). Ideally, you should try to think of a way to strap in the carrier if you can. Maybe bring something like bungees but make sure she can't chew them from in there.
I usually bring a cheap thermometer so that I can make sure the temperature in the car isn't too hot or cold...It is sometimes hard to tell when the vents are blowing on you but not the bird. Also, make sure you have a blanket to cover the bird with when transferring from house to car and car to house (a large box works in very cold weather). Abrupt temp changes are another stress on their systems because they can't adjust as quickly as mammals when things change too quickly. As a rule, 10 degree+ shifts in temperature should be avoided.
I travel with my cockatoo a lot (9 hours= longest trip) and it is stressful at times---you might want to bring along some qwik stop (can be dangerous if ingested), corn starch or silver nitrate swabs (to stop bleeding) ...maybe a pliers and a towel etc JUSSSST in case she breaks a blood feather from flapping-- in case of emergency--if that were to happen and you could stop the bleeding, I would have a vet pull it BUT, just in case you couldn't, you might need to pull it... (mine hasn't ever broken one in the carrier, but she did flap in there sometimes when she was new to travel, so it will just depend on the stress). If you can get a cage that is big enough for flapping, that would be great because then hopefully her wings wouldn't hit the sides (but if she has traveled in the carrier she has then it's a tough call because any change (including new people/car etc) is going to be stressful. See how she reacts to music if you want-- ask the current owner if the bird prefers a certain genre (you could try playing that and see if she seems to calm down or escalate).
Take turns peeing if you have to stop (one of you wait in the car with the bird while the other goes inside etc). Do not bring the bird inside of places where they use Teflon/PTFE/PFOA...like any restaurant or gas-station...I would try to disrupt things as little as possible.
Mine uses the same travel cage for everything, but it is smart to have 2. I tell mine honestly where we are going each time and she has learned (doctor vs. grandma's vs. home)..
Mine doesn't like looking out the windows and she gets stressed when she does.
ALSO--- I would plan the trip so that you aren't driving in the dark. I LEARNED THAT THE HARD WAY....Mornings and afternoons...dusk and dark are bad (screaming etc).
NO AIR FRESHENERS IN THE CAR AND NO PERFUME/scented stuff! My vet literally told me to return a rental car that had been professionally "freshened" before a long trip w/Noodles because it wouldn't have been safe. She said to exchange it for a car that had sat on the lot (uncleaned lol).
Final thought- ask the current owner what time she goes to bed and what time she wakes up. Find out what this looks like and if she shows any signs that she is ready for bed. I would try to keep that routine as consistent as possible, remembering that cockatoos specifically need 12-14 hours of quiet, dark sleep each night. Cozzzy Covers makes a great cover for cockatoo-sized cages (if you want a cover for when you get her home).
http://www.cozzzybird.com/ --I got mine on Amazon.
Have you traveled with a bird before?