In my experience, clients might want to build a CMS for a very wide variety of reasons, and not all of those reasons are necessarily good ones. Your job is to ascertain why they are really undertaking this project, and to deliver the best possible solution.
To most effectively render a good service, I would suggest the following approach:
1. Determine their business objectives.
What kind of business is this? What do they sell, and to whom? By answering these basic questions, you will be able to approach your solution with those objectives in mind.
2. Determine the objectives of the project
Where does this particular piece fit into their puzzle? Do they really need a complicated thing with a ton of features, or could they accomplish the same thing with a far more simple solution? If you can bring them a better, more efficient, cleaner solution, you will demonstrate to them that you are an expert worthy of their trust (and money!).
I have encountered countless occasions in which someone thinks they want something big and complicated, when all they really need is an extremely simple solution.
3. Offer options
If, after determining objectives, there is not a single obvious solution, you may consider presenting two or three possible options. You need not bog them down in the technical details; you simply want to be able to give them a rough idea of the possibilities, and which ones are more or less complicated and expensive to implement.
For example, I have often been asked to provision a blog for a client that is just now getting around to having one (!!). By showing them two options, the "Cheap, Simple, Basic Features" route via a Tumblr/Posterous etc and the "Powerful, Robust, Complicated" route via WordPress or something similar, I can make it easy for them to understand the differences and the possible scopes.
As with all things in Project Management, it's all about healthy communication. The more you understand where they are coming from, the easier it will be to ensure you are rendering a service that will make them happy.