How much data do u need?

I'm making the jump to a 64gig 4g version, from a 64g wifi.. It's been generations since I had an iPhone, been using my blackberry with its 500mb data plan and never go bove 400, however blackberry compresses stuff so I can't gauge it...

What I wanna know is, I'm gonna basically use my iPad for iMessage, light mapping, and just forum browsing/article reading ... Majority of the time I'll have wifi so... How much data would I need? Rogers has a promo right now where first two months are unlimited and months 3-6 are double the data .. Does this mean the monthly signals or would I have to enter into a contract?

I'd be conscious of loading videos and big pictures, but I don't wanna get the 4g version if it means I can't do squat with it or use up all my 250-500mb during the first 2 weeks