The iPad that is available online from the Apple Store is the iPad 4 - this is where I would buy one, unless there's an actual Apple Store nearby. It's a nicely sized screen, fast and an all round excellent device. The iPad mini is smaller, much lighter and thinner but just as fast and just as good quality.
The mini could easily slide into a handbag whereas the iPad 4 is a bit bigger (we carry it in a backpack). Both have an excellent camera and come with the same Apps preinstalled. I recommend getting a 64gb model as you're likely to want to store educational data on it.
64gigabytes is the largest capacity they make, and should give you enough space to store all your work. If you want to be online while travelling you will want an iPad with cellular capability (so you can put a phone SIM card in and be connected via the mobile phone network).
I would not get an iPad 3 or 2 if you can get the iPad 4, as they are slower than the 4 and the iPad 4's screen is much higher quality.
Feel free to ask about anything else before you buy (or after), we're happy to help!
As for longevity, our iPad devices are as-new and we've been using them since they came out. I'd recommend a clear screen protector to keep the glass scratch-free.
Also, for typing you'll probably want to get a Bluetooth Wireless Keyboard... and a case to keep it safe.