New Iphone 32GB Gs Questions Please...

[wnm0]

I wish to buy one of thses but the o2 tarifs just put me off a little,,So I need a couple of answers please if you have time Thank you..
1) Could I use it on Tesco with there triple top up,, top up £15 get £45
2) Would I still be able to access the internet while on the move..
3) Would I be able to use all the Itune's features while using Tesco tariff
4) Would I have to buy an Iphone that is unlocked by apple.
5) Lastly would I lose any functions and features of the phone by not using it on o2 please..

Thank You, Wayne...