dj hardware and software

Here's my choices for DJ hardware:
1 - http://www.amazon.com/Native-Instruments-Traktor-Audio-Interface/dp/B00FYQFPFU/
2 - http://www.amazon.com/Numark-DJ-Multi-Channel-Interface/dp/B000S5Q2EK

They are both simple and effective.  They both have two audio outputs, one for headphones and one for your speakers.  I would recommend the first one first, but if the reviews turn you off for some reason, go for the second.

As for software, you have a few options.  One would be to continue to use iTunes as your main playlist player, and then get the wonderful VLC media media player
http://www.videolan.org/vlc/index.html
You can then set iTunes to play through the first output to the speakers, and VLC to play through the headphones.  That way you could listen to whatever you want to cue up in headphones.

Another option would be to get Traktor DJ software.  You can try out the demo first:
http://www.native-instruments.com/en/products/traktor/dj-software/traktor-pro-2/demo-manuals/
And if you like it, it is $99.  It has more mixing features, which may be useful, or they may be overwhelming.

I would suggest trying both after you get the hardware and see what works for you.

If you have any more questions or want some help setting any of this up, I'd be happy to help.

Godspeed!