I decided that the most economical solution was to buy a touch screen ereader. That way I could read and write on any PDF. My research (and I did a lot of it, trust me) lead me to the Sony PRS T1. I would have opted for a larger screen, but it was extremely difficult finding a device under $200 AUD that met my requirements (touch screen, PDF compatible, wi-fi, long battery life, lightweight) that was also available in Australia. And a tablet/iPad would have definitely been too heavy and too short battery life. Not to say that I'm disappointed with the purchase: but I would have preferred more options.
I'm quite surprised that there isn't more call for academic-use ereaders: I came across innumerable questions and comments online about the lack of a specific device tailored for academia. But at least I have my, if somewhat roundabout, solution.