I have one in my '51. It's just a cheapo unit from Amazon (I think it cost $39.95). I actually installed it to test a 6 to 12 volt power converter we were developing at the time, but I am so used to having it, I can't see removing it. The hardest part was running the wiring and optical cable from the trunk to the dashboard. I see they now have "Bluetooth" units at reasonable prices that eliminate the optical cable.
All you need is a 6 to 12 volt booster to power the camera and screen (another inexpensive item from Amazon). I don't have backup lights on my car, but bought a backup light switch that mounts on the shift linkage to feed power to the system when in reverse. These days, you don't have to worry about polarity because everything like this comes in a plastic case with two wires and are completely insulated, so you can just connect the wires properly and go. Also, there is usually no good place to mount a screen that is of enough size to be useful, so you're on your own.
I have found that the amperage draw on any modern accessory is very low. I also have a cassette player (with a motor and all) that draws .37 amp. The camera and screen is less than that. (I have a prototype power converter on my car with a detailed LCD display that tells me what's going on.)