[*] good formating/display of code [* ] support for Epub formats and non-DRM books is a huge* plus [* ] quick transition between pages [* ] not too heavy
In general, what deters me from e-readers is the lack of a nice way to annotate (with handwriting). I get the problem with EPUBs but it should be fine with PDFs.
Generally, EPUBs are a problem for technical books where I want the figures to properly match the corresponding text. Plus some figures are bigger than the screen (have to be so). And reading PDFs is a problem for the same screen size reason...
The best I can think of is Sony's gigantic DPT-RP1 [1] and it seems to have perfect handwriting support. But at almost 800€, I really don't call this affordable.
Depending on your use case, most standard e-ink (preferably e-ink carta) devices should be just fine. I'd ask you to temper your expectations, however, if your workflow is heavy in PDF reading.