I have used the Fujitsu Scan Snap S5100 in law offices where they hold up well to regular heavy use. They can do duplex scanning and come with a complete copy of Adobe Acrobat Standard, which lists for $299.

I know a number of offices that use the Brother multifunction machines for scanning jobs similar to yours with good results. They have 35 page ADF's and use the excellent Paperport scanning software. These start at under $200. Some models are networkable.

Note that for optimum speeds, it is essential to scan in black and white at lower (100x100 or so) resolutions.