More or less anything from Focusrite, Presonus will serve you well. They'll get the job done until you care about subtle details (which may never be necessary), and might still be fine even after that.
If you're using Linux, the only thing to be sure of is that the device is described as either (1) "class compliant" or (2) "works on iPads" or (3) both. Linux users have much to be grateful towards the iPad for, in particular the ban on drivers meant that USB audio interface makers really had to get their act together to ensure that their devices worked with a generic USB audio driver (just like the one on Linux).
