512207 CMS programmer

I need to have a CMS build, which should be highly customisable (If that's even a word) I'm a Danish webdesigner, and I want to be able to offer CMS and hosting to my clients - I want the same system to be able to handle a customer who rents out appartments, another one who sells cars, posibly someone who runs a information site etc etc... Basicly, I want a system where I as the manager, can set up a site to fit the customers needs, from an admin panel, a system where it will be simple to program new addons, if a new type of client apears. Hope this is sufficient description to start with - Also, if this pans out as a good cooperation, it wont just be a one time job, but posibly a long term contract.