Interesting. This is one of those things where experience just really helps. A friend of mine is a Creative Director at a firm and I asked her for advice for a startup I funded and she explicitly advised against an agency at the stage it was at. I'm pretty sure she'd advise against this as well. Instead, a single contract freelancer is probably a better play than an agency for this.

Over time, I have come to value my network very highly. Definitely helped me avoid a lot of missteps.