In my opinion i dont think that S2 is overpriced becouse it comes for me the best program for dj-ing.I would recomend that you first decide your softwear and then choose your gear.
If you choose MIxtrack Pro wich is great for beginers you would need to upgrade traktor (if you are in Europe you will get Traktor LE with Mixtrack Pro, in america it comes with VDJ).You could also buy MIxtrack NOT pro and Native Instruments Audio 2 wich is better soundcard then soundcard in MIxtrack pro.
You said that you will only mix house disco etc.. so for that you would not need jogwheels. I think that Twitch is very god but you are also going to need to buy traktor if you are going to use twitch with it.
So at the end i would buy s2.
I hope i helped you.