What is a purchase deed or title deed ?
Is a purchase contract deed filed at the Notary, the last agreement deposit or purchase contract execute by deed.

Must I request a copy of seller´s title deed ?
Yes you must request a copy because in this document the majority of the legal references about the property are recorded, is useful for read up on with exactly data.

What is the principal purpose of the purchase deed ?
The main purpose is to transfer the legal estate in the property from de seller to the buyer.