Have you looked at Django lately? They basically give you everything you need including a full-featured with JQuery admin back-end. Their template system is extendable and swappable for alternatives if you want more flexibility. Not to be a nay-sayer, but this sounds like a tough sell when there is so much people can get for free.

Can you beat Squarespace in terms of features, performance, support, etc.?

If you think you might be able to give it a competitive edge pretty similar to ExpressionEngine give it a go:) It's difficult to offer more without having more details like costs etc.