Is it better to marry or to pay off the debt of my deceased father, given that I have three years of study remaining in a non-Muslim country?

If it is possible to combine marriage with paying off your father's debt, then the former is more preferable. However, if that is not possible, then you are not obligated to pay off your father's debt from your own money, except as an act of kindness. Marriage is permissible, and may even become obligatory if you fear for your religion and chastity. But if you do not fear hardship, then paying off the debt is more preferable as an act of filial piety towards your father, and you will be rewarded for it.

If the debt involves usurious interest, you are not obliged to pay it; in fact, it is not permissible to pay it. Only the principal of the debt should be paid. However, if the payment is to be made from money left by the father, then the debt must be settled first before the inheritance is divided or before any marriage expenses are taken from it.