Don't change wfConfig.php. If anything, use the Options Page to check your system's configuration (bottom of page link) for memory_limit. It's probably in the neighborhood of the Allowed Memory Size in your error message.
I'm guessing you have a bunch of plugins that are hogging up all the memory.
When my scans finish, they show how much memory Wordfence used, and it's well below the Max Memory I've allocated on the Options page.