IPad ...inputting stereo line audio from Sonar DAW

I have to ask , why not use just do it digitally ?  use WiFi,
or dropbox ? even iTunes.  

If you want to record direct to an iPad the ios version of garageband can handle the job, though you will need an iOS compatible interface and the "camera connection kit" which is basically a lightning to USB adaptor cable. I've recorded into Cubasis on an iPad Air using my RME interface and it works fine.

Another way to do the job would be if you have a computer that's connected to the internet. Just put iTunes on that then add bounced audio from Sonar to the iTunes library (use a USB stick or CD as the transfer medium if you don't have the DAW on a LAN) then sync it into the iPad.

Yes. With RME's iOS version of Totalmix controlling the interface itself. It has to be said though that RME's interfaces are much more flexible than most of their competition's. The UFX can even be used as a stand-alone recorder or a live mixer.

Basically you need to track down an interface the manufacturer states works with iOS. As I understand it, iOS implements much of OS X's core audio functions so an interface which will work in OS X without a third party driver is likely to also work in iOS. Core Audio itself can also give quite decent latency performance without the need for things like manufacturer-supplied ASIO drivers.

To be honest, unless you have a requirement to use an iPad as a full DAW, using itunes to transfer stuff on and off an iPad has the advantage of being much simpler and far cheaper unless you have the hardware already, in which case itunes is merely more convenient.