This might be useful as a guide.

http://shop.vodafone.co.uk/shop/mobile-phone/iphone-3gs-32gb-black

This is the one I have

http://shop.vodafone.co.uk/shop/mobile-phone/sony-ericsson-w715

How much you are going to use it, is the big issue

For example this is the current price plan for the one I use.

http://shop.vodafone.co.uk/shop/catalog/compatiblePlanListView.jsp?dependantSkuIds=sku1020042&phoneType=post&skuType=phoneSku&se

300 minutes if you are careful would probably suit you.

However, it probably would be best to buy the phone and then Pay as you go, until you get an idea of how much you use the phone.