I have now purchased the Scan2CAD software. Note that since many of my scans are made as PDF files I purchased the more expensive version of the Scan2CAD software which handles pdf formats. I'm still getting use to the software but believe that it will do a decent job. The documentation that comes with the software is good. My only complaint is that for the cost of the software I still need to use yet another software package to convert any pdf files of that were made from CAD models but not from scanned images.
Agreed that one needs to start with a good scan. When I scan paper prints I have fairly good control over the original document quality and can make adjusments to the resolution to create a fairly good starting image. Scan2CAD has built-in features to help clean up the document before the conversion is made and also has some OCR capability to convert the scanned print notes into text (provided the initial image is good). ... However... I receive a lot of customer scanned drawings that are often scanned to low resolutions and are of extremely poor image quality. With such a poor start, it is seems near impossible to make a conversion without putting a lot of effort into cleaning up a print. Often times it's easier to make a new print.