1. You can use Site Sculptor by itself if you are happy with the presets it already has. To have more control, you can make your own presets based on either a CSS Sculptor preset or a DWT plus a menu writer menu.
So, the workflow for your own custom one would be to create a CSS Sculptor and CSS Menu Writer preset first and save them. Then add a new site design to Site Sculptor and you will be able to choose from your saved presets from those other products. This works the same if you want to use a DWT. Just make sure you have the DWT in your selected DW site and it will show up as a selectable in the site design add wizard.
2. A DWT will give you more flexibility while a CSS Sculptor preset will give you more structure. A DWT will allow you to do whatever you want basically, while CSS Sculptor is a little more strict since you have to use that product to do what you want.
3. For CSS Sculptor, you can't have any more regions. For a DWT, any additional regions besides the menu, footer, and content will automatically be added as CMS regions.