Best web architecture for website clusters

I am planning to start a project to build small websites of similar structure to individuals and small organizations. Which is the best architecture that suits: 1. I should be able to offer at cheaper rates 2. Able to scale one site if needed 3. Able to roll out the changes to all these sites when basic template changes 4. Security (I may have to support e-commerce at small save to certain customers) 5. Able to manage all these sites for features like ad-support 6. I would like to build something that is portable (no lock-in and proprietary technology/architecture)

Any pointers that may help in finding the right approach is welcome. (Note: I will be building and maintaining these sites)

Thanks, GL