Should I ask my Seller for a Seller Property Information Statement?

Yes!
As a Realtor you must provide the best, up to date advice to your client regarding the disclosure of defects on their property.
There are two types of property defects, latent and patent defects. We will describe both and why disclosure can be important.
Patent defects are those defects or problems that cannot be seen and are unknown to the seller. These cannot be disclosed as there is no knowledge of them by the owner of the property. This could include something that is hidden in or behind a wall, something unseen. You cannot disclose something you do not know about, can you!
A latent defect is one that is known. A leaky basement where the water has been mopped up is in that category. An old oil tank that is buried, and that the owner is aware of is also a Latent defect. According to common law, it is the responsibility of the seller and his/her agent to disclose these latent defects to potential buyers.
Controversy has resulted in recent years from lawyers who state that disclosing the existence of a defect in a Seller Property Disclosure Statement may cause a lawsuit or admission of guilt on behalf of the seller and thus some liability if the buyer chooses to sue over the disclosed item or situation. While that may be the case, it is a certainty that if the seller does not disclose, in writing to prove the disclosure has occurred, that the seller will be proved guilty of not disclosing something that may effect the value of a property. The agent's responsibility, proven many times in court, is to relate to the seller that this disclosure is important and will reduce the opportunity of a law suit if done properly. The best venue we have is a Seller Property Information Statement. So we recommend it be used, always.

Do I need a witness for a real estate contract to be valid?

Yes and no!
No, you do not need a witness for a real estate purchase or lease agreement to be valid in an Ontario, Canada court. The witnessing is simply a way to determine if the person actually did sign the contract. The Consideration is actually the "seal" on the contract. In this case is it is suggested that the seal is initialled through or the signatory actually ends his signature on the seal. This in effect shows the client has sealed the contract and knows what that entails.
Yes, it is always preferred to have a witness for all signatures to prove that the person who signed actually exists adn agrees to the terms they signed for, but it does not negate a contract that is properly sealed.