Note, first of all, that the Xone DX is a midi controller with a soundcard, not a mixer, so you’re mixing internally with that.
If you buy used, assuming you check it out to make sure it works, you will get a much nicer mixer. I mean hell, we’re talking about an Allen & Heath Xone vs. a NUMARK MIXER. Let’s be serious here.
The 4d is similar to a Xone:92 but has 3 bands eq instead of 4. Still has the great filters and sound quality, and is Traktor Scratch Certified, which is quite handy. Nevermind the fact that it’s actually been around for a few years so most of the bugs have been worked out. The only real advantage I see in the Numark is the jog wheels. Having had an S4, I’ll say that it’s definitely nice to have high resolution jogwheels, but it really depends on your workflow as to how valuable that is to you. Plus, I’d much rather use DVS than midi jogs anyway, which makes the 4d a better option in my eyes.
Another thing to consider, though it will leave you without transport control (no jogwheels, though you could easily use DVS) is a DJM 900. I’m not sure how much they regularly go for, but I saw one on craigslist locally for about $1350 (posted price) recently. It may not tickle your fancy but it is a solid option in that price range, ableit without transport controls (though you can midi map the entire mixer, just about). Currently I’m running a DJM900, an x1, a midi fighter, and two Technics 1200s and it’s my favorite setup thus far easily. I previously had an s4, as I mentioned earlier, and while that was a good controller, I found internal mixing mode to just sound thin and lack clarity. Now, a DJM900 and two TTs will cost you around 2 grand, even used… so this may not be realistic. But, it’s something to consider, especially if you find good deals on the equipment and have the patience to do so.
I’ve also seen used DJM 800s go for about a grand. If you don’t have a soundcard then it’ll probably come out pretty close to the 900 in price with the card though.
Alternatively, just as another option, I do have a used Mackie d.4 Pro which I’m looking to sell for around $600 shipped w/in the lower 48. Traktor Scratch Certified firewire soundcard which is actually a more useful card than the DJM900 card, 2 analogue filters, and a send-return bus. With the money you have left over you could buy a used pair of Techs or possibly CDJ 800s. I know it’s cheesy to advertise in a thread like this, but it seems like it could be a good possibility for you given your budget. PM me if you’re interested.