If possible, adding a table to the current database is the most straightforward. This way, you'll simply take advantage of the database connection and simply need to code appropriate PHP/MySQL to create the desired recordset. Looking at some blocks may give you insight into appropriate coding.

If you have another datastore and need to integrate, you'll need to setup a connection string with the same parameters that you'll find in the C5 config.php file.

first of you don't need a second database, just add another table to main db.
and about creating data grid at the dashboard it's pretty easy, you can check codes for other dashboard single pages already have grid, like search users.

Jordan Lev has created a demo CRUD package with dashboard: entry.https://github.com/jordanlev/c5_boilerplate_crud.