Since, in a gig like this, you can never enforce your own conditions (like saying you won't amend the design on every whim) you have to let the tools enforce this for you.
If I was ever to try this again I would opt for an easily user-editable, hosted solution. Wordpress will be ideal: http://en.blog.wordpress.com/2012/11/14/wordpress-for-cities/ You can cast your role as advising them on how to run it: information architecture; doing the limited number of graphics and showing people how to use the editor. Your role is not to continually re-design (just customise the template), nor to populate the whole thing. You'll also not have to put up with a 2am phone call from the Mayor to say your site is flagged as having malware and is littered with anti-city comments. Wordpress will deal with that for you.
I have used Drupal (and CiviCRM) for other sites and they are phenomenal tools...I just think for a 6,000 grouping they are overkill. And remember if the city wants personalisation, user accounts, billing, consultations etc. online then they really should be paying for someone to develop it for them (perhaps using those tools).
Hosted Wordpress will also help you see whether they are ready to run their own online affairs.
"One Architecture, One OS" also translates as "One Egg, One Basket".