Which version of Omnipage did you try. No OCR is 100% accurate.
Are you actually looking for a Scanner that is quick and cheap?
Why do you need to turn those Scanned Documents into "Editable Documents"? Can you not just leave them as the Scanner produces them. If the files take up too much space ~ Scan at a lower resolution.
Are you looking for a, space saving, way to Archive your Invoices? HM Customs & Revenue may not be happy with a system which is not based on Hard Copy. In other words ~ you are advised to keep the evidence.