I came at this way back in the epaper epoch by getting an Illiad2 from IRex Technologies (a large epaper device with a watcom digitizer), I've since gone through an iPad, iPad Pro, and currently use a Surface Pro 4 for this function.
The essential bits are exactly smooth drawing on it and for wide market penetration low cost. I have been really impressed with Microsoft's ASIC which makes drawing on Surface almost lag free.
Sony on the other hand has a history of weak business models, I was amused by their selling of the books made available by the Gutenberg project for their original e-book (as an example).
The product I would find really interesting (and would buy at least one of :-) Would be an e-paper tablet that I could draw on at a reasonable PPI (200+) that was US standard paper drawing area, built in wireless network and presented to my computer as a 'printer' on the network. I would use it by printing to it from my phone or my laptop and I would use it to review things and write notes on them (sort of like Drawboard PDF) and then either push the annotated prints into an archive or delete them once I was done with them.