As far as the role of the notary public is concerned, s/he is called to ensure that the transaction meets the legal requirements which does not mean ensuring a good deal for the buyer.
Furthermore, opposite to what many people believe, the notary cannot guarantee the absence of legal issues such us illegal works. The notary will check planning permission for example, but he will not inspect the property to ensure the absence of illegal works.
Appointing the notary public is a right conferred by the law to the buyer, it is highly recommended making use of such right. It is never advisable using a notary public suggested by the vendor or real estate agent.