Look first at relevant experience, not the size of the portfolio. Ask for a couple of projects that resemble your stack, and then ask which engineers actually built it. A serious vendor will introduce you to the people who would work on your project. Answers that name nobody at this stage almost always mean you are talking to a reseller.
The paperwork needs more scrutiny than the proposal. Three clauses do most of the work: assignment of intellectual property, non-disclosure, and notice periods and handover. Every artifact must transfer to you as it is paid for, including source code, designs and build vs buy software infrastructure as code. Look closely at language that keeps reusable components outside the transfer, as it is usually the dependency that makes switching painful.
Ask where their numbers come from. An honest estimate arrives with the assumptions behind it, a breakdown by feature or module and a range rather than a single number. A fixed price only makes sense when the requirements are stable and documented; otherwise the supplier pads the number and you fund the buffer regardless. Time and materials shifts that risk to you, so it outsourcing eastern europe requires a sprint cadence, demos and a budget cap.
Process matters more than headcount. Establish how change requests are handled, laravel web development company who defines done and how testing is organised. A team can show you a live build at the end of each sprint. Acceptance criteria in writing remain the only reliable protection against an argument at delivery time.
Finally, real estate development agency think about the handover before it becomes urgent. Ask that the code repository stays on infrastructure you own from the first commit, and that the documentation is refreshed in every sprint. A vendor with nothing to hide will agree quickly; hesitation here tells you most of what you need to know.