i phone advice please

[greenthing]

Hi
I'm looking to buy a a newer  phone I've had my old phone for about 6 years now
and has started to play up
Last week My OH washed my mp3 player and it's nice and clean but is not working now  :'(
so I've been looking to get  an all in one phone camera and mp3

I was looking at a second hand iPhone 3g 32gb on eBay
but I'm not sure if it will work

I'm on a vodafone contract and wanted to know if
its just a case of swapping over the Sim card or will I have to change contract to use it

I get some MB usage in my contract  but I  have never use them
what is a jailbreak phone ?

Thanks greenthing

[greenthing]