If you need to create a custom checkout with your xml you should start by creating a checkout for eCart for linkpoint. You can then modify the code on the confirm and success pages to work with your new setup.
When it comes to using the values from eCart you can just make use of the bindings for things like the item's name, price, cart subtotal, etc.
This will be a custom implementation though that will require you to do a bit of hand coding and some testing to get it worked out properly and it will not be supported.