is the story of it interesting? how long is it?
do I have to buy it before I start my playthrough, or can I finish me2, then buy it, then play it with my file and save the progress?
Well, it will tie into ME3 and have some effect on the story. I'd say the events are worth knowing about at least. I want to say it's maybe 1-2 hours, but I honestly don't remember how long it took me.
And no, you can buy it and finish after your playthrough as in ME3 you just straight up transfer an ME2 save file instead of transferring an endgame save. I would probably even recommend playing it after you complete ME2's story, though you can technically play it whenever.