If deceased father's debt is not paid off and the eldest son is buying stuff for his own house, is it permissible? And buying new clothes for him and his family?

If you are asking about using the heritage/inheritance, before settling father's debt, then it is not allowed.
But if he is buying from his own money, then, it is Okey.
On the other hand, settling father's debt -if he did not left any heritage- is not the eldest son sole responsibility.