Hopefully I'm not misunderstanding, but if you are talking about a flatbed scanner, any of them should work since the software that comes with them generally saves the files as pdfs or jpegs that you would then import into flexi. We have both Flexi 8 and Flexi 10 which we use to import files from a standard HP LaserJet Pro 400 (printer, fax, scanner - nothing exciting but gets the job done for anything under 12x14 inches)
I would instead look at what bed size and scan quality you need and talk to a sales rep