The major issue with converting ATM into some open source machine is the display and touch screen.

Touch-screen:
Transparent Glass I2C Interface 7" Multitouch Projected Capacitive Touch Screen FN070AV01.

Luckily Raspberry has capability to communicate using I2C.
I2C is a bus communication protocol similar to RS-485 (very popular in security systems) or CAN-bus (very popular in automobiles).
They are a little bit more complex that RS-232 or TTL that is being used for peer-peer connectivity.