I have three sites that I would like to impliment EE. 

They are all standards compliant XHTML/CSS/JS with a little flash here and there.

There are Three basic page designs.  A home page with a large header, a main content with a left and right gutter, and a main page with no gutters.

How easily is it going to be for me (a relatively sharp web developer) to get EE installed and my CSS template rebuilt?

You can easily implement any design into EE thanks to it’s fantastic template system. Apart from the documentation, there’s a wicked guide over here that can help you understand how the template system works. If you’re a sharp coder, than you won’t have any problems.

I’m not a pro coder and I’ve managed to use a WordPress template for my EE site.