I think you may find the EQ settings, especially ViVA and BBE MP to be worth your while. I came from a Rockboxed iPod Video 5.5g, and liked the sound of the Cowon S9 even more.
While all have long battery life, the J3 is the longest. The D2+ and C2 have resistive screens, while the J3 and S9 have capacitive OLED screens, that look much nicer and are easier for touch navigation.
The J3 is your best bet as far as long battery life and SD expansion. It supports some 64GB cards.
I've seen no other player that has better battery life than the Cowon J3, and while a Sony player may suffice, I think you will be impressed with the EQ options on the Cowon players, after trying it out. The interface is also very customizable on the J3 and S9, and lets you install other GUI's for each function. Some support album gestures to play/pause prev/next and change volume by interacting with the album art, and others add playlist features and such.