A state machine for loan repayment sounds overkill until you're 3 months in and a stakeholder says "what if a loan is partially paid but also overdue but also flagged?"
Build the state machine. Day one.
A lesson I learned last week: when an idea drops into your mind, build it fast. Because the moment you think of it, 5, 10, or even 20 other people are probably thinking about the same thing too, and some of them may have already started building.