iOS & Android Payment Calculator

I need a clean, cross-platform app that lets a buyer punch in the purchase price of new or used equipment and instantly see the resulting monthly payment and effective interest rate. The user experience should be as simple as a single-screen form: they enter the cost, tap “Calculate,” and the results appear immediately.

Behind that simplicity I’ll supply several preset variables—term length, down-payment percentage, residual value, fees, and any other factors we agree on—so the calculations happen automatically without exposing those fields to the end user. I’d like an admin area or a secure JSON / config file where I can update those numbers without shipping a new version of the app.

Key requirements
• Runs smoothly on both iPhone and Android (a single codebase with Flutter or React Native is fine, as are separate native builds).
• Calculates and displays:
– Monthly payment
– Effective interest rate
• Tailored to equipment financing, not generic auto or mortgage loans.
• Professional, lightweight UI that matches my brand colors (I’ll provide the palette).
• Final hand-off must include full source code, build instructions, and test builds for each platform.

If you’ve shipped finance or loan-calculator apps before, especially with equipment-specific logic, that’s a plus. Let me know what framework you prefer and how long you’ll need for a working prototype.