You should have used the money (1)/ for paying your debts (2)/ instead of buy a motor cycle. (3)/ No error (4)

Answer

3

Explanation

buying will replace buy because - a Gerund (- ing Verb) is used when a Verb comes after a Prep. Here, instead of is a Prep. Hence, Gerund will be used.
Look at the examples given below:

Hence, instead ofbuying a motorcycle is the light usage .