Look first at proven experience, not the size of the portfolio. Request a couple of engagements that match your technology stack, hire nuxtjs developers and then ask specifically whether those engineers are still with the company. A serious vendor will put you on a call with the engineers. Answers that name nobody at this stage almost always mean the delivery team is not the team you were shown.
The paperwork warrants more attention than the sales deck. Three sections matter more than the rest: assignment of intellectual property, the NDA, and termination and software development cost estimate handover. All the work product should transfer to you on payment, along with designs, scripts and infrastructure configuration. Look closely at any clause that leaves so-called reusable libraries outside the transfer, because that is often the part you cannot replace later.
Ask where their numbers come from. A credible estimate arrives with a written set of assumptions, web application development for edtech a task-level breakdown and an explicit range. A fixed-price contract only makes sense when the scope is genuinely frozen; otherwise the provider adds a risk premium and you pay for uncertainty either way. flutter developer hourly rate billing shifts that risk to you, so it requires visible weekly reporting and a spending cap.
How the work is run beats team size. Ask how change requests are handled, who signs off on a feature and how quality assurance works. A mature team will be able to show you a live build at the end of each sprint. Written acceptance criteria are the practical protection against the it-was-never-in-scope conversation.
Finally, think about the handover while the relationship is still good. Require that the source repository stays in your organisation from the beginning, and that the documentation is refreshed in every sprint. A partner who is comfortable with this accepts it without argument; hesitation here tells you a great deal.