Begin with proven experience, not the length of the client list. Ask for a couple of projects that match your domain and your stack, and then ask who actually wrote that code. A solid partner is happy to connect you with the tech lead. Vague answers at this stage usually mean the delivery team is not the team you were shown.
The paperwork warrants a slower read than the pitch. Three sections matter more than the rest: intellectual property assignment, confidentiality, and termination and handover. All the work product should transfer to you on payment, erp development company together with designs, scripts and infrastructure configuration. Watch for wording that keeps framework code in the vendor's hands, because it is usually the dependency that makes switching painful.
Find out how much does it cost to build an app the estimate was built. An honest estimate comes with a list of assumptions, a task-level breakdown and an explicit range. A fixed-bid deal only makes sense when the scope is genuinely frozen; when the scope is still moving the vendor pads the number and you pay for it anyway. Hourly billing shifts that risk to you, so it demands a sprint cadence, demos and a budget cap.
The delivery process matters as much as team size. Ask what happens when the scope changes, who writes the acceptance criteria and how testing is organised. A mature team will be able to walk you through running software rather than status reports. Acceptance criteria in writing stay the only reliable protection against the it-was-never-in-scope conversation.
Finally, consider the handover while the relationship is still good. Require that the code repository lives in your organisation from the first commit, and that documentation is updated as part of the work. A provider confident in its own work will agree quickly; hesitation here says a great deal.
