You kind of found your answer but just in case you really need a server for not only handling the push notification (for telling your app in Newsstand to check the new issue and download it in the background) but also for subscription and in-app purchasing.
Every time you launch the app you should check few things before you refresh the UI. You should check if the user is a subscriber or has user already bought the issue so you can show view/download button instead of Buy button.
Check out these tutorials if they help to understand these communications better but believe me you will end up with one of those services like Urban Airship (Which I think UA is the best option because it handles all in one not just push notification):
http://ios-blog.co.uk/tutorials/how-to-make-a-magazine-app-in-ios-part-i/
this one is more focus on server part:
http://ios-blog.co.uk/tutorials/how-to-make-a-magazine-app-in-ios-part-ii/
http://www.viggiosoft.com/blog/blog/2011/10/17/ios-newsstand-tutorial
http://www.raywenderlich.com/2797/introduction-to-in-app-purchases
And as always this is the best place for start (it has everything you need):
https://developer.apple.com/devcenter/ios/newsstand/
These are some resources to help you in developing app for Newsstand but still there are not enough people out there to share their experience.
Best of luck.
Edit: Add these two links two this list:
http://www.marco.org/2012/10/11/the-magazine
http://www.marco.org/2012/12/12/the-magazine-future-of-publishing