Pop a drive in as a secondary (USB or SATA), dDW would wipe the drive top to bottom. Perhaps offer options similar to what CCleaner does (1,3,7,35 passes).
Now the tricky part, when done, it creates a PDF certificate detailing the drive's specifics (SN, size, etc.), slaps a logo on to it and saves it for the customer.
When I went googling "drive wipe certificates", I found this and got the idea:http://how-to-erase-hard-drive.com/images/LogoTop.png