2 questions about a vodafone contract?

I'll be ordering my blackberry curve 8900 on a �30 a month tariff. What I'd like to know is, if for some reason mid way through the contract (24 motnhs) I can't afford to keep paying it, is there anyway i can stop it and change the phone to pay as you go? also it says it will be delivered in 2 - 4 business days but in your experience how good are vodafone with on time deliveries? Thanks :)