Since you are thinking about doing the exact same thing I did a few months ago, feel free to send me some questions in a PM or something if you need more help.
Ask Dead Mouse Five for some gear questions.. I'm sure he could tell you a thing or two..million..
DJM900 makes perfect combination and goes along very well with the S4 and Traktor. You should get one and include it into your Mothership setup.
Yes, with S4 itself should be able to handle most of the work, however due to inaccuracy sound issue in S4's knobs when playing music as standalone device, i prefer to hook it up with external mixer like DJM for better accuracy. With external mixer like DJM900 means plenty more rooms for creativeness and improvement in my mixes.