A Traktor Scratch certified SoundCard. Go to the NI website the cheapest one (I think audio 2) is NOT certified and will not work! I would recommend one of their packages as they also include the control vinyls you need anyways.
A good bunch of cables and you can go from there.
However you would have to use mouse and keyboard to manoeuvre thorugh Traktor, so I recommend something like an NI X1 or Novation Dicers at least, to get you a better level of freedom.
So this I think is the cheapest package:
http://www.native-instruments.com/en/products/traktor/digital-vinyl/traktor-scratch-a6/
And I would go and add this on top:
http://www.native-instruments.com/en/products/traktor/dj-controller/traktor-kontrol-x1/
Or as an alternative you can go for this mixer and save the hassle of cable setup and audio interface:
http://www.native-instruments.com/en/products/traktor/dj-mixer/traktor-kontrol-z2/
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