O.K so I'm writing a iPhone travel guide, you purchase a subscription to a travel guide for 3 months, it downloads a fairly hefty database and for 3 months that database gets updated weekly with new stuff.

Now what I'd like to do is make the user enter their email address as a one off action before they purchase their first guide, for China say. The purpose for doing this is

1) To allow me to contact the user by email when they add a note/tip for a particular place (the app will allow them to send notes & information to me)

2) To Uniquely identify who has purchased the subscription so that if they wipe their device and reinstall the app they can plug the email address in and pickup their subscriptions again. Or so they can use the same subscription on another device they own.

My concerns are

1) Will Apple allow the email method of restoring functionality to a second or restored device?

2) As long as I tell the user what I'm using their email address for (aka I won't sell it to anyone else and use it for X purposes) will it be o.k to ask for said email address?

And as a side note, can I tack the devices unique id onto my server comms to track devices or is apple going to through a hissy fit about that as well?