Fair enough. We are working on our documentation, but others have a 10 year head start so it takes a while to be able to do the same.
Not sure what the question is? It won't use the disk unless you use disk-intensive services like web proxy cache or insight reporting.
ARP is automatic, or I don't understand the question very well. Sorry.
Insight reporting is nice, enable NetFlow with local reporting... and skim the plugins list as there are a few other interesting things, also see:https://github.com/opnsense/plugins#a-list-of-currently-available-plugins