Audit plan is definitely required. ISO/TS16949 asks for an annual audit plan.
On the other hand agenda is optional.
You can include sub-processes to be audited during an audit as an agenda which will help auditors to gain some knowledge about the process in advance and prepare accordingly.
Eg. In Quality Control and assurance process, agenda can be like auditing of Incoming inspection, Layout Inspection, Inprocess Inspection, SPC, MSA. (Details of which activities these process performs i.e. is to be audited)
This is what we do and our internal auditors find it useful, since they now know what is to be audited. And this helps the audit process to work smoothly.
You can define what matters to you most based on your organizational requirement.
Thanks,
Amit