Start with relevant experience, not the size of the portfolio. Ask seo agency for software factories a couple of engagements that resemble your technology stack, and then ask specifically whether those engineers are still with the company. A solid partner will introduce you to the tech lead. Evasive answers at this stage usually mean you are talking to a reseller.
The contract deserves more scrutiny than the proposal. A few clauses carry most of the weight: assignment of intellectual property, confidentiality, and notice periods and handover. Everything produced must transfer to you as it is paid for, including designs, scripts and infrastructure configuration. Look closely at language that keeps so-called reusable libraries with the vendor, since this is frequently exactly the piece that locks you in.
Find out how the estimate was built. An honest estimate arrives with the assumptions behind it, offshore php developers a breakdown per feature and a best case and a worst case. A fixed-bid deal is only reasonable when the requirements are stable and documented; otherwise the supplier pads the number difference between symfony and spring boot you fund the buffer regardless. A time-and-materials model puts the risk on your side, so it demands visible weekly reporting and a spending cap.
The delivery process matters as much as team size. Find out what happens when the scope changes, who writes the acceptance criteria and how testing is organised. A team can demonstrate a live build at the end of each sprint. Written acceptance criteria are the only reliable protection against the it-was-never-in-scope conversation.
Finally, consider the day you no longer need this vendor before it becomes urgent. Require that the repository stays on infrastructure you own from the first commit, .net consulting services and that documentation is written as you go rather than left to the end. A partner who is comfortable with this says yes immediately; a long negotiation over it says most of what you need to know.
