I’ve seen the kp3 used by a lot of people. I don’t know if you really need it for trance/house though, maybe for live dubstep remixing.
As for the SCS, I’ve read only good things about it but I’m not sure it fits your needs (“hotcues and such”).
If it was me, I’d try to look into something like the Kontrol X1 or the new A&H Xone:K2 as a way to combine the effects and the hotcues into one unit. Remember, with the setup that you want, you’ll need 4 USB ports. Also, when you DJ in clubs you’ll usually have very limited space to setup your gear between the CDJs, club’s mixer, etc.
And I saw that you were thinking about the new Numark N4. Decide on what’s best for you, especially if you’re a beginner. A 4 channel controller might be overkill at this point (since you’re not getting much more than the Mixtrack Pro). Are you really going to use the 2 extra channels?