I've done various applications, some in Google Play for clients with logon authorization, custom and OAuth2. The sending of an email with attachments are not hard, again I have published applications on Google Play that do this already.
We could review your design and resources and even upgrade the look and feel with Google's Material Design if you like.
The only thing that is unclear is the type of authentication that you are currently using.
Would love to discuss more.
Best.