Apologetics and Upanishads
There's the Milinda Panha. It records a dialogue between the Buddhist sage Nāgasena, and the Indo-Greek king Menandernda_Panha). English Translation available here. It's not considered a core text by all Buddhist disciplines, but it is one of the oldest surviving apologetic dialogues between Buddhism and Hinduism.

Another interesting text is the Buddhist critique of the Advaita Vedanta as explained in Michael Myers' Comparative Theology