Kobo have a quite plain Linux, run on iMX processors, and are very easy to modify/script, and last but not least, to unbrick in case of severe problems. In many models, the internal memory is an SD card, so it can be expanded easily. Then you have also external SD.
On the software side, try koreader. It's an open source reader, developed for eink devices. It works very well with PDFs, and can do intelligent text reflow, even on raw scans of books, via a sort of OCR. The nice thing is that it doesn't convert bitmap to text, but seems to split long lines of text in shorter sections, then rearranges the pieces on the screen, following font size options. It can be instructed to work as expected on multi column pages. Very nice! Andrea
