How much time would I need to prepare before enlisting to make sure I have the best chances of passing selection?
It really depends on what your current fitness level is, however, generally speaking I tell guys who are modestly in shape to give themselves at least 8 months to get ready.
When you're stateside, how often are you with your family?
You can expect to be gone a lot even when you are home. My first year I was gone around 300 days, it slows down to around 200 after that.