I would say it depends on your budget, timeline of the project, etc when it comes to choosing which route to go down.
Custom CMS :
Takes a LOT longer than pre-coded CMS
Better security ( depending on the experience the coder has )
Any potential exploits haven't been made public
More flexible, and you get exactly what you need
Pre-Coded Solution :
Doesnt take very long
Generally a lower level of security, as they are constantly being picked at by hackers. Exploits are public. However with a decent server setup, and a couple of modifications these can be fixed.
I may be able to help you with this project, if your interested, throw me a PM and we'll talk.