"PaperTrack" is one such application. You can read more about it here:
http://www.document-management-software ... ricing.htm
There are a few other document imaging and management software out there but the price isn't cheap.
The thing about zonal OCR (OCR an area) and then using that to index (name the file) is that your zonal area will have to be the same for all invoices.
There are other strategies that you should look at. Consider bar code recognition. Bar Code fonts are plenty and free on the web. You can print labels using bar codes, create your own tables or databases and then now you have a ready tracking system. Most highspeed scanners (Fujitsu, Canon) are enabled with barcode recognition.
Another route is to create a MS Access database of your invoices creating a naming convention. Scan the paper using the scanner to auto-name the images. Now you have a database with which you can look up invoices.