I did a quick spreadsheet on it

Basicly calculated with costs over 24months, and the prepay with the 100mb data plan, with txt 2000
What im not sure about is can you stick addons onto plans, say txt2000 onto the $40 a month plan?, when i go into vodafone next ill ask, but if i can get an answer before hand, that would be great
Data, how much do you use on average a month, is 100mb enough, atm i have an oldschool nokia, doesnt even have a camera so its hard to say, but when i used the girlfriends iphone 3g at work, would just surf webpages and a few apps, but can see me doing more than that if i owned one myself
With Facetime, ive read you can do it via WiFi, but lets say you couldnt how is that data managed, via your monthly data allowance or is it charged like a phonecall with a fee per minute?
Like i said if i can get an answer before heading into a Vodafone shop that would be a bonus