What customer documentation will I need to obtain and where do I send it?

In most cases you won't need any.

In some cases we may need various proofs, you will be made aware of what documents are required at the condition of acceptance. This is done on a case by case basis.

Dealers are required to obtain proof of identify in face to face sales so you should obtain a copy of the driving license (photographic ID) and retain this for your records. This is a requirement of FCA.