Honestly DJing with the iPad is not the end to all things and sooner or later she will want to DJ with a PC or Mac. Hence getting a controller that can do both is a good thing (and the S2/S4 can).
Also a good thing if you buy into an S2 now is, that she will not outgrow the controller quickly and the thing should last quite some time.
However buying used in that case is not what I would recommend, the new S2 Mk II has loads of features geared towards using it with the iPad version and the Mk I does not. So get the newest hardware and get it new so it has warranty and everything you need.
As for headphones I would recommend to get at least the Zomo HD-1200 which are reasonably good and some decent computer speakers for training (like M-Audio AV40).
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