Date for delivery

It's possible to add a date selector for delivery in the checkout?

Yes. You can go in the menu Display->Custom fields and create a new custom field of the table "order" and of the type "date" (this is only possible in the Business edition). That will display the custom field on your checkout so that users can enter a date.

I can use the Business License for more than one site?

The license is for one website (+test and alias).