Start with relevant experience, not the size of the portfolio. Request two or three case studies that resemble your stack, and crypto igaming platform development then ask specifically which engineers actually built it. A serious vendor will introduce you to the people who would work on your project. Evasive answers at this stage almost always mean you are talking to a reseller.
The contract warrants a slower read than the pitch. A few clauses carry most of the weight: assignment of intellectual property, the NDA, and notice periods and handover. Everything produced has to transfer to you once invoices are settled, including designs, scripts and infrastructure configuration. Watch for wording that keeps framework code in the vendor’s hands, since that is often the part you cannot replace later.
Ask where their numbers come from. A serious estimate is accompanied by a written set of assumptions, a breakdown per feature and a range rather than a single number. A fixed-bid deal is only reasonable when the requirements are stable and documented; when the scope is still moving the vendor adds a risk premium and you pay for uncertainty either way. A time-and-materials model puts the risk on your side, so it needs visible weekly reporting and a spending cap.
The delivery process beats the number of developers. Find out how change requests are handled, why mvps fail who signs off on a feature and what the QA setup looks like. A well-run team can walk you through running software rather than status reports. Acceptance criteria in writing remain the only reliable protection against the it-was-never-in-scope conversation.
Finally, plan for the day you no longer need this vendor at the start rather than at the end. Ask that the code repository lives on infrastructure you own from day one, and that the documentation is refreshed in every sprint. A partner who is comfortable with this will agree quickly; resistance at this point says a great deal.