@pulper, you don't seem to be using the information you are receiving here to narrow the focus of your project. In Reply #30 it all seems just as wooly as it was at the start.
As I see it you have been presented with 3 principal options
1 connect the existing RS232 to the Arduino using a suitable interface such as a Max232 chip
2 bypass the existing RS232 and connect the Arduino to the ADC in the black box
3 bypass the RS232 and the ADC and use the ADC within the Arduino
It's up to you to choose, and then if you need more assistance give us the relevant technical details
...R