I haven’t used Borderlands on a mini but you should be fine in most cases (it still works on my old iPad 2). You might wish for a larger screen if you have a lot of different audio files opened in Borderlands, but if you’re planning on using it primarily for live input I really don’t think it would be an issue.
I’m pretty sure any class compliant interface will work. I use a Steinberg UR-22 mkii and have never had an issue (though that M4 does look very nice…I hadn’t seen that before).
As for MIDI clock from other hardware, in my case I’d just send a MIDI clock into the MIDI port on the UR-22 and that’s that. BUT…I don’t think Borderlands currently supports MIDI clock sync?