To become a Buddha must a person be born in a highest caste in their society? To become a Buddha must a person be born in a highest caste in their society? Or if someone is born in a low caste, is he unable become a Buddha?
In the Digha Nikaya in Mahāpadāna sutta, the Discourse of the biography of the Buddha, and in Acchariya Abbhutadhamma sutta we get to read about the life stories of the seven Buddhas: Vipassi, Sikkhi, Vessabhu, Kakusanda, Konagamana, Kassapa and Gotama.  The full life story of Vipassi Buddha given in this sutta explains that the incidents discribed are a Rule (Dhammata) for every Buddha. 

When a Bodhisatta (Buddha-to-be) fulfils all the Paramitas in the last life of before his birth in human world he will be born in the Tusita Heaven. When the time approaches, various signs appear in the ten thousand world systems. The Devas of all worlds assemble in Tusita heaven and request the Bodhisatta to be born in the human world as a human being to become a Buddha. The Bodhisatta will not reply until he has made the five great investigations. (Panca maha vilokana – divine foresight) regarding time (Kala) continent (Dipa), place of birth (Desa) clan (Kula) and mother (Matha).

Kala- Time: It should be a proper time. It should not be more than one hundred thousand years of human life span and less than one hundred years of life span. If the life span is too short people will have excessive defilements. Because of excessive defilements people to not comply with the advice given by the Buddha.

Dipa- contement: Only in Jambudipa / India the Buddhas &amp; Pacceka Buddhas are borne.

Desa- place: (Madhyadesa) – a region in Jambudipa / India

Kula- clan: Khattiya or Brahmana.  The Buddhas belonged to all the high castes, to the Kshatriyas and Brahmanas. It is another of the Rules (Dhammata). So the question of "if someone is born in a low caste, is he unable become a Buddha?" would not arrise.

Matu- mother:  The Bodhisatta Mata in her last birth must not passionate. She should have to practise five precepts without violating them for her life. She should not have more than (7) days life after the birth of the Bodhisatta.