Although I'm a Kindle user, I'd have to agree that nook seems to fit your needs.
The Kindle might be over-featured for you, though you already seem comfortable converting stuff with Calibre, which is generally the biggest hurdle re: using public domain books.
For the purposes of buying newer books, Amazon does tend to have the biggest selection and usually offers the most publisher promo freebies, which you may or may not be interested in (they're mostly evangelical Christian fiction and soft-core erotic romance, but occasionally a thriller or historical gets promoted).
However, nook wi-fi is only $149 US, and can also borrow e-books from the library without jumping through extra hoops, assuming yours has any. And apparently you can read almost any e-book from the B&N catalogue for an hour by bringing your nook into their store, and they sometimes offer free coffee and such to B&N eReader users.
You should probably take a look at the thread Nook vs Kobo purely as a reader
, for people's impressions of the pros and cons of each device.
I also suggest signing up for a B&N account and getting the free classics they're promoting right now
. They're excellent editions with bonus notes and commentary, and they'll be changing the selection every Friday until mid-September. If it turns out the nook's not for you, you can always read them on your computer.