DEFINITELY call your current carrier and buy the Mexican rider for the bike, pretty cheap and a must do unless you can afford to write your bike off. The horror stories will curl your hair. And if you are going very far, contact you medical carrier for some help and guidance, I have only second hand stories, not actual facts on the health insurance but I would not chance it. The air ambulance insurance is also a good idea if you can find a carrier to cover that part of the world. Air ambulance a must if riding a motorcycle in Mexico, YOU WANT TO COME HOME if injured.
No matter what any of these cost, they are a must if you are injured, bike is not rideable etc. Do not chance this, it can break you.
Also I never heard you could buy Mexican insurance coverage at the border, maybe you can, But who would want to chance this. Everyone thinks dealing with an adjustor for a major company is a problem. This has the potential to give you a heart attack.