If you already have a computer, I'd just play live and record with that. Reaper is very capable of running live sound (set for imperceptively low latency) + recording any live inputs and post fx to multitrack. I run live sound for full bands with it in addition to it being my live guitar rig. Simultaneously recording all the multitrack the whole time.
The iPad is a great remote control for running your computer in live sound situations but it's a bit anemic for actual processing. Additionally, ios is extremely watered down and basically acts like a governor on the thing with the main goal of extending battery life.
There ARE some very cool looking options starting to come around for tablets but if you already have a computer the iPad will not seem like a very serious option in comparison. It's been a great remote control with remote desktop though (iTeleport in my case).
If you decide to play with the iPad directly, you'd need an iPad DAW app. It would be self contained and you'd transfer your files after recording. The challenge is getting just about any USB device to talk with the iPad. The functionality to connect to a specific USB audio interface needs to be written into the ios DAW app itself. Ios by itself doesn't seem to have the basic ability to put its own pants on.