Hardware:
With duplex required, it will be difficult to find an inexpensive scanner. A couple of models to look at are the Panasonic KV-S6045W and the Ricoh IS450DE. These are both lower-volume duplex scanners with good speed. The Panasonic is moreexpensive, but it is also more rugged and as better paper handling. I would lean in this direction if I were looking for a 4-year lifespan.
Software:
Both of these scanners come with a software interface, but you should probably look into Kofax Ascent, which is a scalable capture solution. You can output to just about any front-end you decide to go with. On the less-expensive end, you may want to look at Informatik. They have document management utilities from back-end to front-end without per-page charges. IMR Alchemy is a product we have used, as well. I've heard that they've really improved it in the past couple of years, so you should probably look into that too.
Reference:
http://www.1st-in-scanners.com/
http://www.lawcypress.com/
http://www.panasonic.com/
http://www.ricoh.com/
http://www.informatik.com/
http://www.imrgold.com/
HTH,
Tom