CarloK

This is not true. Inside the feeder housing is everything needed for the filament transport, the only external component is the stepper motor which is similar in all current Ultimaker printers.
 
The UM3 firmware is old and from before the CC-core release, but I don't expect problems in that aspect, the printer will accept the printcore.
 
What will be a problem are the material definitions; these consist of two parts:
1) A file on the printer with compatibility settings, temperature (for material change and probing) and a bunch of settings for the Material Station.
2) Slicing parameters in Cura
 
The printer files from 1) are fixable, but the major material tweaks are in the Cura slicing from 2) which I have not enough info for to fix. 
Not tested, but the simplest tweak would be to have the UM3 identify as an S3 printer. Then the printer will accept all S3 compatible materials and PrintCores and in Cura you can slice correctly.
On the printer you have to modify just 1 line in the configuration file:
In /usr/share/griffin/machines/9066.json change "Ultimaker 3" to "Ultimaker S3" and reboot the printer (for how-to see this thread)

The S3 has a slightly larger build volume, so in Cura modify the S3 printer dimensions by copying all settings from the UM3 printer (Cura menu Settings/Printer/Manage printers and then 'Machine Settings')

The API supports up to 10 camera's. For the requirements: plug it in the front usb, and if it works then you are good.
I don't think there are free USB ports inside, this also might differ on the S5 variant you have (v1 has the robot logo, v2 has the 'U' shape logo on the sides).
 
The usb supports a hub, you might even move this inside the housing.
 
Then, recently I learned about this brilliant low tech alternative: stick a mirror to the wall opposite the camera. An IKEA mirror seems to work great. 😎