In a breach of contract action, should all the evidence be presented?

Under Evidence Code § 1453 a signature is presumed to be authorized and valid if it is validated by a notary public. This will allow the admission of the signatures which will make the D look stupid if he says that is not the contract. Ask him to agree to its admission and, if he does, simply that the parties have a stipulation that this document represents the agreement between the parties.

A notarization doesn't validate an otherwise unenforcable contract or have any bearing on whether it's got the essential elements of a contract. All it does is verify that certain signatures, either yours, the other party's, or both, belong to the person(s) purportedly signing the document and prevents them from claiming their signature is forged. You can wait til trial is impending, when the court requires the parties to stipulate (agree) on n facts and on documents that are introduced at trial, but it's unlikely that if the other party is contesting the contract's purpose or performance, that they'd agree. A better strategy is to propound contention interrogatories and/or requests for admission to the other party regarding the contract's execution, effect, and performance, to elicit what they're contesting and why, if you don't already know, and to require them to produce and name any documents, facts, and witnesses that support their contentions. Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.

The notarization of the contract will only help you to authenticate it. In order to prevail in a breach of contract case, you also need to establish the defendant's breach of the contract, and the planitiff's resultant damages.