The walking her should give you a good indication of how she will react to things, but remember- even on the ground she is going to react to how you react. Make sure you are confidant and calm about everything you encounter from the very start. Don't let there be anything negative coming from you, down the lead line and into her head. She trusts you to keep her safe. If you make no mind about a car whizzing by, or that flag flapping in the yard, most likely, neither will she :) I personally would also make sure she is walking with me, not slack dangly off behind me. When she is next to you, you can watch her more- her ears, her eyes, listen to her breathing etc. You know your horse best, what her signs are.
Finding someone that has a horse that's been where you want to go and can give your girl that One Time through showing her the ropes... and as long as the walking through and one time with a buddy works, I would just go from there.
Best of luck!