I've been using flyspeck which requires php. So far, so good. The previous suggestion to check out opensource is a great one also.
However, there is one thing to consider when putting any CMS or any program. From time to time, there will be security holes that have to be patched because of the app itself or the underlying PHP. Who pays in that case?
Also, because we have too many lawyers and not enough chum, you might want to put a clause in your agreement that you are not responsible for site hacking due to third party apps. For example, there was a hole in, I believe, phpbb a while back. One line of code had == instead of ===. The result was it was open to code injection. Two things can hurt you there. (1) the client gets hacked and hires Dewey Cheetum and Howe to sue you for negligence (2) You end up doing unpaid work to patch the app.
Most of the CMS systems are pretty simple for the end user. If you are dealing with technically challenged people, you might want to add some time for training into your calc.
You might want to brief the end users a bit on basic SEO principles. ie. "be careful of linking to bad neighborhoods".
If they have their own hosting, make sure the CMS will run. I ran into a case where a client's host would only allow php to run in safe mode which was useless.