Just thought I'd add my two cents. If you've already got two xone 1d's (wicked score btw!) then you will probably want a mixer. I would consider a mixer with analogue (ie it can run without a laptop) capabilities because they are very handy for just having an ipod plugged in in case of emergencies.
As for the sound card, you might be able to find a mixer with a built in audio interface but then remember that you'll have to take your mixer to any event that you play at. If you have a separate sound card then you kit will be much more portable and customizable.
As for the program, I recommend Traktor but only because it's the only one I've used. Plus you'll get great support from this community with Traktor.
Traktor + the xone's will give you plenty of effect control to work with so don't worry about the EFX 500 or 1000.
All the best!
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