There are dozens of inexpensive solutions. A friend of mine has an 8-track recorder with two input channels, which saves the tracks to an SD Card. He bought it on special at Sweetwater for $150. You'll need to buy an SD card adapter for your iPad if you go this route. You could buy a USB-to-lightning port adapter for your iPad, and run your USB interface that way. I bought an Alesis 8-channel mixer for around $150. It has two output channels, which would work perfectly for live demos and live performances - provided you have access to 110 v outlet. It also has 48v phantom power for your condenser mics. You'll be stunned at how good your sound quality is. Below is my one-take testimonial.
I think AKAI has a 4-input interface that doesn't require an external power source. Again you'd need a USB-to-lightning port adapter, and you'll probably get the most mileage out of your iPad battery if you record with a dynamic, rather than with a phantom-powered condenser microphone.
This does not apply to Pro-Tools interfaces, which are proprietary to the point of being ridiculous. If you have a Pro-Tools interface, it won't work with anything except the software developed exclusively for that interface. To run it on a PC, I needed 200 GB of space available. It's a good system, but I think with Pro-Tools, you'd do your best work on a desktop Mac or PC. I would avoid using smaller computers - even laptops - with this system.
The cheapest solution, given your current resources is to add the GarageBand app to your iPad. The only hardware you'll need for the most basic recording is your Apple earbuds. These must have a microphone on the cord leading to one of the earbuds - mine is on the right. Drop the earbud and mic into the sound hole of your acoustic guitar. You'll need to play with the recording volume to minimize distortion before you begin recording, and of course, you'll need to arrange the cord so it doesn't interfere with your playing hand. Use the other earbud (if necessary) to keep time with the metronome feature on your GarageBand app while you record. Once you've created/assembled your guitar loops on the timeline, you can add the vocals. Pan all of the accompaniment tracks that you need for proper vocal timing to the left. Insert your left earbud into your ear. To avoid excess noise, drape the cord between the mic and the earbud over the edge of your hand (between the thumb and forefinger) so that the mic is facing you. Hold the earbud mic 12-15 inches from your mouth when you sing and keep your mic hand as steady as possible. You want to hold it like this because any movement along the cord will make a scratching sound in the mic. My laptop crashed a few months ago and I wound up writing songs on my iPhone GarageBand app, using the method from above, just to keep the music going. I have another friend who records all of his music this way.
I hope this answers your questions. Good luck!