To actually answer a couple of your questions...
They work anywhere you can get a signal, internet speed will depend on the type of signal you can get in any given area
Generally Iphone contracts have an internet allowance included.
All Iphone models are essentially the same when it comes to using them
And you'll be lucky to get a recent iphone on a cheap contract without paying a lot up front.
But you do seem to be thinking about buying one just because you want the kudos of owning an iphone when you've obviously not got a clue about them, the more normal way to buy things is decide what you want to do with a device and then pick something that suits your needs.
As above says, there are other, equally as good, smart phones available.