@fausty, we are encouraging apps in FreedomBox even when similar apps are available already. This is encouraged as long as there is at least one developer promoting and maintaining that application. Even if WordPress is be included into FreedomBox, Ghost can definitely be added.
I tried to searched for ghost in the Debian archive and could not find anything. Perhaps the Debian package needs to be uploaded into the Debian archive.
As for running nginx and node.js on FreedomBox, there is no problem technically or policy wise. Although, since we are already running a Apache instance, perhaps the ngninx web configuration/proxying part could be written for Apache to avoid bringing in another web server.
If you are a developer and are looking add this app to FreedomBox, we from the core team would like to help you with that. Let us know if you need any information or have queries regarding how FreedomBox works. See the FreedomBox developer manual over at https://docs.freedombox.org.