Your best bet would be a shop that sold unprotected PDFs, and epubs.
For technical books O'Reilly sells without DRM and can choose between several formats including the two mentioned. Can't tell you about more specific shops since that's all the books I've bought and tried on the M92.
About the PDF annotating support: I haven't used that much but it served my needs: You can draw on top of the PDF and also add textual comments. Then you can export a PDF with all those annotations and read it on a PC.
Quick stylus? Not really, but not too slow either.
Anyway, it looks like Onyx will launch new large ereaders in the coming months so you might want to wait a little:
https://www.mobileread.com/forums/sho...d.php?t=227304