Frankly (and probably heretically), imaging solutions such as EMC's Applicaiton Xtender, are highly commoditized and quite inexpensive. You would probably be better of licensing a product or looking for an open source product that does that specifically.
Document imaging has some specific needs around ingestion that many ECM platforms were never designed to handle. I've just barely started looking into Alfresco, so I can't say whether its architecture would handle it or not, but from the previous comment, you are looking at a lot of work to build a commoditized piece of software (content modeling, UI, scanner interaction, etc).
Travis