Your keyboard
^^^^
This.
It depends massively on what you’re trying to do. Personally I prefer using real mixers to the internal one and generally it’s cheaper for something that will take a beating and keep on going rather tha being a flimsy bit of breakable plastic.
+1
Cheapest controller just for playing about and seeing how you like it is the nano-kontrol.
Issue here is that you’re going to need to spend extra money on the traktor software. Why not just continue using VDJ? There’s still so much you can do with. Focus less on your spftware and fancy effects and tricks and things and actually focus on your song selection, transitions, mix structure, etc. Play with phrasing and the like. This will make you a much better DJ than just playing with effects and cue point juggling ever will do.
Keyboard does work great. Pick up a Apple Mac keyboard and use something like http://www.kbcovers.com/servlet/Detail?no=293
It shows it as a macbook but it works perfectly on the Apple aluminum keyboard models such as http://store.apple.com/us/product/MC184LL/A?fnode=MTY1NDA1Mg&mco=MTMzNzg5MDM