UMPC Portal is the best single source for news on computers with 4" to 7" screens. Your best bet may be the
Everun. It is above your $700 limit and has a 4.8" screen, but has the best battery life in the UMPC space. It runs Windows XP, and I suggest using
FBReader for reading your Plucker files. The
Pepper Pad 3 has a 7" screen and is within your price range. It runs Linux, but should do all you want (FBReader for plucker again). I am very happy with mine. Its battery life isn't all that good though, and it is very wide because of the built in keypad.