If it's a creative job like a tattoo artist or designer you may have a bit of room to go past the usual driving license photobooth picture.
For the test, just play it safe and go telephoto+f/4, and to add a fress but still professional touch use a blurred office/park background instead of the standard studio white.
If its going to be at the fair itself find a spot with enough distance to the background and not many people passing by and create that effect.
I think the best workflow would be to create a paper form that you can print to fill up with their name, email address, billing address and file numbers, edit them, upload them to a cloud server in individual folders long with a bill in a pdf file and send each client the link to their folder via email.
Cool, fast, reliable and easier for both of you.
