I can answer only some of your questions. I don't know Rogers plans, so no advice from me there.
The iPhone can be set to detect new Wi-Fi networks and ask you if you want to join. Even if you have that option turned off (as I do), once you have logged on to a particular wireless network, the iPhone will recognize it when you are in range and switch automatically to it and stop using the cellular network for data.
When you download an app, it is charged to your iTunes account. Either you will have an iTunes gift card credit there or you will need to supply a credit card when you register, against which your purchases are charged. They do not appear on your phone bill.
You can make your own ringtones for the iPhone by a variety of methods, most of which you can find by using the search feature right here on the forum. Personally, I use Garage Band for this purpose, which makes it very simple. But there are other straightforward ways to do it too.