Any question answered...

i'll be on wifi for the majority of time - obviously this reduces the usage, but i still don't really have any concept of how much data is downloaded particularly on mobiles. i've heard opera mini is good for reducing the data usage, will give it a go when this shit phone arrives.

Reset the data and see how much you'd use on an average day.
I only ever used a lot when downloading a movie or tv episodes from iTunes.
Besides, browsing and checking emails is ok - even when you send loads of photos.

I've just checked my IPad and used something around 210mb in a month, because posted dozens of full resolution photos.

With very heavy usage abroad and dozens of photos sent it was something between 1-2gb a week.