For the X5 it's a real good deal, it wooth about 700, and if you comes with the suitcase, go for it!
As for the sounds and patches and everything, you can by some external device (or another keyboard) that you plug your keyboard in, and then plug it into an amp or whatever and you use your keyboard to change to sound of the external device.
There's some keyboard (every recent one can do this) that you can import the sound into your keyboard (you generally have like a certain number of sound premade, and like 127 sound space that you can create)
creating a sound is waaaayyy more complicated, it's near impossible to do for an intermediate keyboardist, I think even some of the greatest keyboardist we know can't do that, but there's many things you can do with a patch already existing like, for a basis, changing the envelope.
It means:
-Attack: When you it the notes, what time will it take to the patch to reach its... real sound. It's more or less like a Fade in thing. (At 0, (like piano or hapsichord or bells etc), the sound will be heard exactly when you it the note)
-Decay: Once the patch as reach is real sound, what will the patch do? If you turn it to max, the patch will sound exactly the same will you have your fingers on the note (except if you have an Aftertouch option)
-Release: Well, after you release the note, will the sound continue or not?
After that, you can play mixing some patch or had other effect that's becoming more complicated. (like Filter LFO, Cutoff, Resonance, Oscilliation, Modulation etc)