Serato on iPad I don’t know about. Traktor on iPad exists but is not the same as regular Traktor and as far as I know the data (cue points and such) doesn’t even transfer over from your PC Traktor to the iOS Traktor.
Best choices in apps for iOS in no particular order:
DJ Player
DJay 2
Djay Pro (this I believe does sync data with it’s – mac-only – big brother)
Cross
All of these allow USB-connected midi devices. Depending on the app they need to be midi class-compliant to work. Some have supported controllers, others will accept any class-compliant device. Some have fixed mappings, other allow you to make your own or adapt existing ones.
Depending on your controller you will have a connector that fits your iPad (be sure to look for the difference between old 30-pin and new lightning connector!), in which case it should power your iPad at the same time. If you want to have a generic controller with just a USB port connected, you are gonna need Apple’s Camera Connection Kit (CCK). This WILL allow you to hook up a controller, but WON’T let you charge your iPad. Also you’d need a controller with external power supply. I don’t think the iPad can power a USB-powered controller (and if, then not for very long).
Hope that helps some.
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