About the push notifications - some apps that support it usually turn it on on first use (just for the app). From what I know, you can turn all notifications off from the Settings menu. I've always had that to off (forgot when, but I had to do it, it wasn't off by default). Never received any AppStore, iTunes, or any other notifcation, therefore I assume no data has been used.
Prior to the introduction of the push notifications feature, I was monitoring the data usage when no apps were running, and it was zero for periods of more than 12 hours. I still think that if you have notifications off globally, no data will ever be used if you don't start any apps.
With Data Roaming off I did quite a lot of roaming around the world, never payed a cent for data usage.