Hey @mglace -- nice to hear you're looking to migrate!
The best way to migrate would be to build your own exporter by forking an existing one... for example the SMF exporter... they work with the importer plugin: https://github.com/akhoury/nodebb-plugin-import
Alternative you could use the Write API, although you'd be writing a lot of the one-time logic yourself to migrate users and posts and such, and I don't know offhand of anybody using the Write API quite like that 😄
You're on track with the SSO OAuth plugin. DFP should work using the adsense plugin... if not, you can ping @baris for help.
Theming NodeBB is one of our strong points, our themes are very customisable. Almost everything can be customised including templates for each page, as well as adding additional styles, etc. Check out the quickstart theme for a shell to start off with 👍