We're getting ready to start moving our Exchange 2013 users to Office 365. At the same time we want to open up Outlook Web Access for everyone outside our network who hasn't been migrated to Office 365 yet.

From much reading it looks like I could accomplish this by setting up a Web App Proxy on the edge and then when users logged in, ADFS would decide where their mailboxes were, either on premise or in the cloud. Am I looking at this the righ

ADFS doesn't decide anything like that, it only provides a token for authentication.  Exchange will direct the users to the correct location, either on-premises or Office 365, if you properly set up a hybrid configuration.  Consider hiring an experienced consultant for this.

Thanks Ed.