Look first at proven experience, not the size of the portfolio. Ask for a couple of projects that match your stack, and then find out who actually wrote that code. An honest provider will put you on a call with the tech lead. Evasive answers at this stage generally mean the delivery team is not the team you were shown.
The paperwork needs a slower read than the pitch. Three sections matter more than the rest: assignment of intellectual property, non-disclosure, and termination and handover. Everything produced should transfer to you once invoices are settled, including source code, designs and infrastructure as code. Watch for wording that leaves framework code with the vendor, because that is often the part you cannot replace later.
Ask where their numbers come from. A serious estimate comes with a list of assumptions, custom development vs saas a breakdown by feature or module and a best case and a worst case. A fixed price works only when the requirements are stable and documented; otherwise the vendor prices the risk in and you pay for it anyway. Hourly billing puts the risk on your side, so it needs a sprint cadence, demos and a budget cap.
Process matters as much as team size. Find out how change requests are handled, hire react native development team who signs off on a feature and how quality assurance works. A well-run team can walk you through a working build every one or two weeks. Acceptance criteria in writing remain the only reliable protection against endless rounds of rework.
Last, node js vs laravel performance think about the day you no longer need this vendor before it becomes urgent. Insist that the code repository stays in your organisation from the first commit, and that documentation is updated as part of the work. A partner who is comfortable with this will agree quickly; hesitation here tells you quite a lot.