No — you don't need a Mexican will to purchase property in Guanajuato. We do, however, strongly encourage you to get one shortly after buying.

When an owner dies without a Mexican will, probating the property so heirs can take title can drag on for months or even years of expensive legal work. With a Mexican will in place, the timeline shrinks to weeks or a few months, depending on the circumstances.

Also note that a will from another country has no effect on your Mexican property unless it's been translated and apostilled — and even then, pushing a foreign will through the Mexican courts takes longer than if the will had been drawn up in Mexico.