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1) No, Ableton is nothing like Traktor, its a sequencer, not a DJ mixing program. As far as I know Traktor is the best program for computer DJs. However, I haven't used PCDJ or whatever since I'm on a Mac, so I can't commment on that.
2) Yes, it's very tempting...you can just hit the Sync button and it keeps it all beatmatched (and key matched) for you, no work or skill needed, that's why I don't like computer DJs 
3) Yes, it's worth getting a MIDI interface. I actually just got an Evolution 449c USB/MIDI keyboard, mostly for using with Reason/Cubase for production, but I was playing with Traktor and it was a lot of fun. You can set any of the controllers or keys to any function. I found the sliders good for pitch, EQ, and volume, the knobs good for the filter and other misc things, and the actual piano keys great for stop/start, cue, EQ kills, punch, kill, etc (if you can remember what's what, or label them)
If you're at all interested in producing as well as DJing then definitely get a keyboard MIDI controller rather than just the knobs (like the UC-33e or X-Session) so you can use it for both.
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-tom
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