Your best wager is a second-tier Android tablet. ViewSonicG Tablet/Advent Vega/Point of View Mobii (all the same thing with different brands) will allow you to load anything in the SD card, be it a Word document, a .cbz comic file... you name it, and read it with an app (possibly you'll need a paid one for the word documents, but you're covered with online and offline manga). It works like a charm, and the OS is really fluid: plus the form factor is quite good for manga.
The drawbacks are the fact that you need to research a little bit in order to install all the good things (it doesn't have Android Market and other apps right away), the legibility of the capacitive screen (though the Viewsonic GTablet has more quality in that respect) and possibly the price point, which would make you need to save a bit.
There are (much) cheaper Android tablets with resistive screens, but I can't give you much insight there. My point is that buying a cheap Android tablet will need a bit of time investment for you to improve them.