You've effectively been given the task of writing a full blown Content Management System. This is a mammoth task that would probably take a lone developer anything from 6 - 24 months to build depending on experience (this based on development time of other CMS' on the market). For instance, the developers of Umbraco (an Open source ASP.NET CMS) are busy porting their CMS over to ASP.NET MVC, work started around beginning of this year and is not expected to be built until middle of next year, and they're some of the most talented devs in the industry.
I'm not doubting your talents, but unless your boss has given you a very large time scale to work to, or you plan on your website builder being extreme basic with minimal features, perhaps building a full blown Website builder or CMS is biting off more than you can chew.
As other posters have recommended, you should perhaps try existing CMS on the market such as Umbraco if you're a .NET developer.
If you do insist on building your own, it will need some serious planning. Look at software architectural design patterns such as DDD (Domain Driven Design), SOLID principles such as Dependency Injection, the Repository Pattern, Persistance Ignorance, Service Layers etc etc. MVC is definitely the right way to go. Also check out Martins Fowler's book Patterns of Enterprise Application Architecture or Dino Esposito's Microsoft .NET: Architecting Applications for the Enterprise or Eric Evans Domain-Driven Design: Tackling Complexity in the Heart of Software
