Is there a Torah source or minhag to make a seudas mitzvah, when one finishes paying off his home in chutz la'aretz?
Answer:
I do not know of a specific or formal custom in this matter.
Although completing one's payment for a home is a personally important matter, and it can bring a person much joy, it is not an actual mitzvah, and therefore will not warrant a "seudas mitzvah."
Yet, making a meal for family and close friends, at which one expresses one's gratitude to Hashem for His goodness, is always a positive thinkg.
Best wishes.