To monitor the system information available in the iLO which port(s) should be allowed on the target machine in order to communicate with PRTG?

First you need to enable SNMP on the iLO and then use SNMP sensors.

Please take look at the sensors we have out of the box for HPE ProLiant servers

https://www.paessler.com/manuals/prtg/available_sensor_types.htm#snmp

but please take a look at the requirements as well, especially this sensor type supports monitoring HP Integrated Lights-Out (iLO) as of iLO version 3. We recommend that you use iLO 4 because this version applies its own dedicated SNMP counters while iLO 3 only forwards SNMP counters from the particular operating system.


Disclaimer:
The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.