I will describe the existing format of our websites and what we want to be able to do below.
We have two websites. Domain 1 includes descriptions etc, but does not process the order. When a customer makes a purchase, they first click on the link to buy now and this takes them to our second domain where our order processing system is located.
Domain 1 will not be an entire website, but will only be one page. Domain 2 consists of a wordpress website and whmcs, but we only need to track whmcs as it is our order system and users from domain 1 are sent straight to the whmcs cart to complete their purchase.
What we need
We need to be able to use Google Analytics to track the sales that are made, allowing us to see where the users came from, what keywords they used to enter domain 1.
I do not think that we need any goals setup as Google seem to claim that this is detected using the ecommerce option within Google Analytics. However, you will need to make sure that our sales will be tracked so that we can see who bought what products and where they came from, what keywords they used and what pages they viewed.