As for lesson readiness, my own inclination is to wait until kids are 4 - 5. There's so much informal musical learning that can happen before that. Even structured informal learning. (Try looking into Alice Kay Kanack's "Musical Improvisation for Children" book and CD; I think it's fabulous!) Get copies of the Suzuki repertoire CDs, start listening every day, start attending group classes and lessons as observers. My favourite beginners to teach are the ones who have been attending classes as observers for up to a year beforehand. They've already learned so much, and they and their parents thoroughly understand the expectations that take root in a private lesson and group class environment.
I'm meeting with a mom & daughter tomorrow about starting lessons in the fall -- the child turned four in April and while they've been observing classes on and off for 18 months, the child has only recently been able to understand that Suzuki violin expectations include not interrupting another child's lesson with questions and trying to get physically involved with the lesson, not squirming and chatting or laughing or wandering around. Nothing wrong with that sort of behaviour coming from a 3-year-old -- it's developmentally appropriate, but it's tough to run a goal-oriented private lesson or group class with a noisy preschooler moving around based on her own bio-developmental prerogative.
So you can get the best sense of her readiness by observing lessons. Not only will you be able to see what lessons are all about and develop a gut feeling about whether the structure will suit her, but by bringing her along you'll get a good sense of whether she can adapt to the behavioural expectations of a private lesson studio.
Where would one find the Suzuki CD's you recommended?