It’s pretty simple. Off the top of my head:
Create your sandbox, install discourse.
Decide if you want mail (you don’t really need it if you’re just testing how to do “stuff”)
Fudge app.yml so mail won’t go out.
Download a backup from live, restore.
Restart.
Check it works.
Clean up the .db (https://meta.discourse.org/t/disk-space-usage-by-postgres/61137/4)
Have at it.
EDIT: My sandbox is just for checking software and such, so I don’t have outgoing mail. In fact I deliberately mangled all the entries in app.yml so that it would never send mail (and I disabled all outgoing).