I am looking at getting these invoices read while being scanned, by an OCR engine added to the xerox scanner we currently use. The other option is to buy a small to midsize scanner that can optically read the invoices when they are scanned and will populate some of the fields in our routing database, automatically.
While we do not review scanning equipment or provide specific product recommendations, we can point you in a direction for making informed purchasing decisions. There are a number of outlets online where you can find product reviews.
CNET.com is one of the more reputable technology review sites. There you can browse a variety of document scanners, read about their features, and read the pros and cons of many of them. Be sure to click on "Document Scanners" under the "Find By Scanner Type" subhead on the top right. Click on each scanner to read its specs. Many pages include descriptions of pros and cons.
Another site you may want to visit for reviews of OCR technology is PC Magazine. They recently posted reviews of several new OCR engines in a feature titled "The Next Generation of OCR."
Using these sites as a starting point, you should be able to get an idea for what would work for your needs.