What are the minimum fields required on the checkout?
At a minimum, we require the transaction details (Amount), plus the contacts Last Name.
This said, the more information you have, the better the matching and de-duplication will be. If you were to just have the last name, you would always end up creating a new Contact as an example.
For EziDebit, the minimum is contact first/last name, phone, mobile, email and full address details.