What is meant by “work across more than one country”? Do you need one interface, or truly comparable data? Those are different requirements. A system can import each country’s label correctly without making the ratings equivalent. I would also ask what happens when a listing import conflicts...
Ask for three separate totals: money due to complete, recurring ownership costs, and costs triggered only by a later sale, change of residency or death. A single “closing costs” figure can hide assumptions. Each amount should state who calculated it, what buyer status was assumed and whether it...