Get Nightfall and Eye of The North to begin with imo.
I'd say go for Prophecies, but the original campaign is nowhere near populated enough to start with these days (not that it's entirely a ghost town, but I could imagine becoming frustrating for a new player). Getting the above two will give you access to all the Heroes the game has to offer as well as the majority of the PvE only skill sets.
If you find you fall in love with the game after your experience with these, I'd highly recommend the other two campaigns for the campaign specific skills and content they offer.
If you can afford all, I suppose go for it, but you might find it best to focus on one storyline at a time. You could always buy the PvP unlock pack to keep your heroes up to date while you work through the stories one by one.
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PS: Check out amazon, their 'Better Together' and 'Perfect Partners' deals are pretty good sometimes. Check these links:
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