Product marketing should understand product requirements and feature velocity at a high level i.e. how hard it is to deliver a solution and how many features can your team deliver in a quarter. That way you can bucketize feature requests into t-shirt sizes; small, medium large efforts. The onus is on product managament and engineering to determine exact costs and timing of features.
To establish credibility with a new team, the first step is understanding the team's need, laying out a vision for how you can best add value, and aligning around expectations. It is important to know the user, the market, and the product so that you can engage with the cross-functional team in a meaningful way from day one. With a clear set of objectives and foundational understanding of the space, you can quickly begin to make an impact on the team.