- itunes... depends if you have older, drm'd itunes music that you never upgraded to 'itunes+' w/out drm. if you do, you're kind of out of luck on that unless you pay to upgrade those files, otherwise no problems.
- the apps won't work- you'll have to repurchase (likely the exact same apps will mostly be available)
- macbook- depends what you want to do.
- you can buy unlocked, or carrier subsidized. up to you.
i would probably look at a galaxy nexus if you're looking for an unlocked phone. you can't beat $350 for an unlocked phone...