the eCommerce database is a good starting point:
ecommerce-database/
for your purposes, you would need the users, orders and order details table
you can certainly get the security assist login portion created before starting with the payment portion. but some of the security assist setup will rely on the payment setup to be complete.
you can start with setting up the security assist pages for login, registration, forgot password and such, so that they are in place to tie in the eCart checkout.