Need a monthly-payment calculator widget for commercial vehicle leasing

We run a leasing company that finances commercial vehicles for small and medium businesses — trucks, delivery vans, minibuses and specialized construction equipment. Right now a prospective client has to call our sales team or submit a request just to get a rough idea of what a monthly payment would look like, and that friction is costing us leads, because people want to play with the numbers themselves before ever talking to a manager. We want an interactive calculator widget embedded on our website that lets a visitor select the vehicle category, enter the vehicle price, choose a lease term in months, and set an approximate down payment, then instantly see an estimated monthly installment.

The tool doesn't need to connect to any external credit-scoring service or issue a binding offer — it should apply a formula we'll supply, based on residual value, an interest markup that varies by vehicle type, and the chosen term, then present a clear, well-formatted result with a short breakdown: down payment amount, financed amount, monthly payment, and total cost over the term. It has to work smoothly on mobile, since a good share of our visitors compare offers from a phone, and stay light enough not to slow down page load on our existing site.

We're open to a simple front-end-only widget with no backend, as long as the math and rounding rules are easy for our own staff to adjust later without touching code. Looking for someone who has built similar finance or pricing calculators before.

What we need:
— Fields for vehicle type, price, term, down payment
— Instant monthly payment estimate with a clear breakdown
— Mobile-friendly, lightweight, no backend required
— Configurable interest logic and short embedding instructions