Starting with Sitefinity

Hi, 

We want to build an application for a company to register its employees to events. 
We don't have previous experience with working with Sitefinity so what would be the best approach for this thinking ahead on maintenance and further extensions? 
Here are a couple of questions/decisions that we consider at the moment:
- Build the application using Sitefinity modules or create the DB separately and build our own widgets to access it from Sitefinity? And if we use sitefinity modules what is the proper way to make relations between tables and how do we handle many to many relations?
- How should the application admin account be handled? Use the Sitefinity admin dashboard or create a portal for an admin as well?

Thank you,
Maria