Kasumi's loyalty mission is great, but it's only one mission, so unless you've still got stuff to do that you can take her along for, you're better off getting a mission pack.
Of the two mission packs you can afford, Overlord is the best. Arrival is only really worth getting if you want to see the whole plot, since the mission is fairly basic but it sets up the beginning of ME3.
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"I'm Commander Shepard, and you're standing in the biggest library in Citadel Space. Look me up."