Benjamin,
Installing the CMS database locally should be ok but I would have concerns installing the Audit database locally and using SQL Express.
Depending on the level of Audit being used the database will grow and will require some database management. Also if you decide to move the Monitoring database to Audit, this will add to the overhead.
Also make sure to use SQL Express 2012 64bit ADV download with Management tools, see link for download below.
Download Microsoft® SQL Server® 2012 Express from Official Microsoft Download Center
Regards,
Ajay