BJP
I misread/misunderstood your post the first time I posted, the second time I posted, I understood what you have for the installation.
There is absolutely no code or other reason you cannot use the existing bus with the green screw provided by the manufacturer.
The question about the rating of the circuit breaker is a very good question.
Here are some links that will help one to determine the type of circuit breaker that will be required.
Link1,
Link2 (this is a good link), Link3,
Here is the link for Google to many good links for the topic of OCPD