There are NoSQL (NewSQL) databases that are fully ACID compliant that you could consider. I would use one of those to handle the transactional CRM data. There are simply too many benefits using these compared to traditional relational databases:
- Much better performance
- Schemaless
- Some let you remove the ORM completely and uses the created objects automatically
- Some have integrated web server with REST/JSON support, that would be nice for you since you will work with web apps for the end user.
The ACID part is very important if you will build a CRM. I once build a CRM system that uses a NoSQL database and the performance made it possible to add features we never would have considered if we had used a traditional RMDBMS.
I like the idea that you should put the media and documents into a CDN and then refer to them from your database.
Your open source requirement could be a bit of a showstopper though.
I wrote an article on the subject that you might give some advice in the topic of selecting a database: