Good luck finding a decent condition SX/SX2 for 600 unless you’re super lucky. (SX2 esp.) Unfortunately, it’s hard to find a controller with a standalone mixer in the price range he is in as I’m in the exact same boat. The NV is cool with the attached screens, but you’ll have to go used to get under 600 and no stand alone mixer.
Here’s a comprehensive list for you to look at including the NV depending if you can do without a stand alone or not.
4Trak, N4, NV (used), Terminal Mix 8, Pioneer DDJ SR, Kontrol S4/ S4 mk 2 used, Vestax VCI 400, Allen & Heath Xone:4D, American Audio VMS 4.1, Denon MC6000/MC6000 mk2 (Used), Gemini g4v, and finally, Slate 4.
They each have differing features, most of them close but not an exact match to the NS6 except the 4Trak (Software) and N4 (Build quality). Also note that most of these are native mapped for Serato and/or are software dependent. Feel free to ask if you have any questions, because as I said before, I’m in the exact same boat and will be buying pretty soon.
TO answer your post, if you want a modern (Not discontinued or old) controller that works plug & play with virtual Dj, get the G4V. It’s true 4 deck controller (no deck switches like the Terminal Mix 8 or DDJ sr for instance) that is mapped for Virtual DJ & Traktor. Though it doesn’t have the stand alone mixer feature, you’d be hard pressed to find that in almost any DJ controller and saty within 600 usd.