As for using non-Traktor hardware with Traktor software, you might want to keep in mind that Traktor has moved away from supporting third party hardware (natively). Any support (mappings, drivers) comes from either the third party manufacturers or places like the Traktor Bible site.
It is, at the very least, unpredictable what will happen when a new version of Traktor hits the market. At this point in time I would not want to risk a serious investment in hardware that might not work with my preferred software in the near future. Again, no guarantees this will happen. Unfortunately there is absolutely no guarantee that it won’t happen either!
I own an DDJ-SX and it’s a good, well-built controller. But I have owned lots of Denon gear over the years (including MC6000, SC2900, X1600 and DN-S2000 recently) and with the introduction of the MCX8000, that is going to be my next controller.
Clearly support for Serato, fully midi, so can work with anything else. Sure VDJ will have a mapping soon, as I am hoping/guessing will Mixvibes (for Cross). It has screens as well and uses Engine software so you can use it stand alone (actually with just a USB-stick with only tracks on it!).
There is a lot to choose from 😀