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Start with proven experience, not the size of the portfolio. Ask to see three or four projects that sit close to your domain and your stack, custom software development qatar and then find out which engineers actually built it. A solid partner will introduce you to the people who would work on your project. Vague answers at this stage almost always mean the demo work came from somewhere else.
The contract needs more scrutiny than the proposal. A few clauses carry most of the weight: assignment of intellectual property, confidentiality, and exit terms and handover. Everything produced has to transfer to you on payment, along with documentation, pipelines and deployment scripts. Be careful with wording that leaves so-called reusable libraries outside the transfer, since it is usually the part you cannot replace later.
Find out how the estimate was built. A serious estimate is accompanied by a written set of assumptions, a task-level breakdown and an explicit range. A fixed price only makes sense when the specification is complete; in any other case the vendor prices the risk in and you pay for uncertainty either way. Time and materials moves the risk back to the client, so it needs visible weekly reporting and a spending cap.
Process matters as much as team size. Find out what happens when the scope changes, outsource retail ecommerce development who writes the acceptance criteria and how testing is organised. A team can demonstrate running custom affiliate tracking software rather than status reports. Written acceptance criteria stay your only real protection against an argument at delivery time.
Before signing, consider the end of the engagement before it becomes urgent. Require that the source repository lives under your account from the beginning, and that the documentation is refreshed in every sprint. A vendor with nothing to hide says yes immediately; hesitation here says quite a lot.