Some agencies will take on any project, regardless of their experience. Instead, find a partner with domain expertise and past work in your industry.
It's counter-intuitive, but adding more developers may not speed up your project. It will, however, add a lot more to your budget without guaranteed results. Right-size your team instead.
After discovery, you should have a detailed set of project requirements, timelines, and a budget. This is the only reliable way to measure progress and approve payments.