Begin with domain experience, not the size of the portfolio. Request two or three projects that match your stack, and then find out which engineers actually built it. A serious vendor will introduce you to the people who would work on your project. Vague answers at this stage almost always mean the delivery team is not the team you were shown.
The contract warrants more scrutiny than the proposal. Three sections matter more than the rest: intellectual property assignment, the NDA, aso consulting services and notice periods and handover. Every artifact must transfer to you as it is paid for, including designs, scripts and infrastructure configuration. Be careful with wording that keeps framework code with the vendor, since it is usually the dependency that makes switching painful.
Find out how the estimate was built. A serious estimate is accompanied by the assumptions behind it, a breakdown per feature and fintech development company an explicit range. A fixed price only makes sense when the scope is genuinely frozen; otherwise the provider adds a risk premium and you pay for it anyway. A time-and-materials model shifts that risk to you, so it demands a sprint cadence, demos and a budget cap.
How the work is run matters as much as team size. Ask how change requests are handled, who defines done and how quality assurance works. A mature team can demonstrate a live build at the end of each sprint. Clear, written acceptance criteria stay your only real protection against endless rounds of rework.
Finally, plan for the end of the engagement before it becomes urgent. Require that the repository stays on infrastructure you own from day one, and that the documentation is refreshed in every sprint. A vendor with nothing to hide accepts it without argument; a long negotiation over it outsourcing usa reveals most of what you need to know.