If you want it for reading manga, that's definitely the reader you need. I do have a DX for that purpose, and it is wonderful. My manga PDFs are around 20 to 30 Mb each and the DX is able to open them quite fast.
Regarding textbooks, everything up to a A4 in size will be displayed ok; a book like Krugman's Macroeconomics
will be squeezed and the font size may look too small. Also remember DX doesn't display colours.
I agree that the internal storage is not enough, but I think a DX really worths it, unless you can find another ebook with a display of that size.