Most go for the least expensive they can find but that may not be the best for your booth. First, you need live view. All currently available SLR cameras support live view and so do many older cameras. If you have an older camera that does you may want to start with that. If you plan to offer video recording in your booth then research carefully because many lower end cameras that offer video recording can’t be switched back to still photo mode through software but require a human to turn a dial. Don’t be lured into that high megapixel camera because of its promise of higher quality, most photo booth strips have an individual image size measured in the mid hundreds not thousands so an image size of 5200x3500 has to be downsized to 500x425 before printing on a 2x6 photo strip. Using a smaller file size will give you faster transfer from camera to computer and faster printing. With a photo booth you will also want a camera that has an AC power adapter because with constant use an SLR can burn through batteries fast causing delays and lines while you change batteries every 30 minutes. You will also want to have extra USB cables on hand in case one gets damaged.
Every photo needs light and there are many options to choose depending on your needs. If you don’t offer video then flash is a great choice because you won’t need blinding bright lights shining in the subjects eyes all of the time. You can choose to just use the on camera flash that is built in or go with a separate external studio flash for a better quality of light. With an external flash you will also need one that does not require batteries for the same reasons as the camera. Be sure to set your timing to allow the flash the few seconds it needs to recycle before the next shot and just like the USB cable you will need to have spare sync cables to go from the camera to flash. If you are planning to offer video then you will need a constant light source instead of flash. LED and compact fluorescent are great options because they don’t get hot like regular light bulbs do.
A photo booth printer needs 3 major things, they need to be fast, reliable and hold enough consumables to not require changing every few minutes. All of that adds up to a professional grade dye sub printer. These printers can go all night on a single roll of paper/ink ribbon and print out 2 2x6 photo strips in seconds. Their cost per print is low as well and they can go for years without any problem. Just like cameras, they need a cable to connect to the computer so bring extra.
The brains behind it all, the computer you choose is a balance between power and size. Make sure you have the speed needed for all of the booth options you plan to offer. For example, video can require a lot of disk space for storage so you may need a large hard drive. Also, a really slow processor may result in a sluggish booth experience for the customer. Touch screen or arcade buttons are another thing to think about. Touch screens bring with them a great deal of power and ease of set up but arcade buttons can be more cost effective and durable.
I have mentioned this several times already but there are a lot of wires that connect all of this and one can get damaged at any time. The best camera, computer and printer in the world won’t work if the $3.00 cable is broken. Take spares of every cable type. It is money well spent.