I think in your situation it is far better to keep your cat indoors. The people and the traffic could pose a huge danger to letting her outdoors.
Getting a big cat tree is a great idea, as well as getting her used to a harness and taking her on supervised trips to the outdoors. Be sure to provide her with plenty of things to keep her occupied inside, and she'll be fine.
What sort of patio do you have? Many people, myself included, have enclosed their patios to provide a safe area for their cat. Maybe post a picture of your patio, or the type of patio you have and people can give you some suggestions?