As somebody who does both steampunk events and renaissance faires (both as a patron, and faire worker, depending on the event), I do tend to prefer vendors to stick with the theme. I.e. its great if you sell both steampunk and ren-faire stuff, but I would prefer to see mostly the ren-faire stuff at a ren-faire, and steampunk stuff at a steampunk event. Patrons (or playtrons if you prefer) will wear whatever they please, but depending on how inappropriate the costume is, you will be talked about.
Now for a photo booth, you can disguise the camera, but it is a lot of work. And note, a large format camera design like my steampunk camera, is still about 200 years in the future of the period set by the faire. I've thought about doing a general artist gig, where you have an easel set up and put a live view screen on the canvas, and disguise the camera with paints and rags draped over it. But with only 2 weeks to showtime, now may not be the time to do redesigns.
I would probably not do the flash paper, while it would be cool, since it might get dry enough that it could be a potential fire hazard. I don't recall if you need a permit to use it. If you do it, you would have to factor in the cost of flash paper into each photo. I could imagine using the Arduino to fire off the flash paper, and it doesn't even have to be used for the photo, you can use normal flash, but close enough that most people won't notice.
Obviously you need to factor in how long your camera and flash batteries last, and make sure you have plenty of extras. Bear in mind you will need to either have enough to last you all 3 days of the Labor day and Columbus weekends, or you need to have enough chargers and amperage to be able to charge the batteries overnight. You don't want to have to wake up every 2-3 hours to change batteries if you only have one charger.
You presumably have already thought to have enough paper and ink to last each weekend. Given the dust in the area, I would suggest making sure you have backups if/when some gear fails.
If you are doing special effects on the computer (tinting, etc.), make sure they are automated enough that you can just pop in the card, or upload from the camera, and have everything happen. You mention internet sales, my sense is in general you won't get that many, since the essence of a lot of these photo booths is instant gratification.