I don't think the fact that you'd be their first client is why you should run away screaming, I think all of what you said here is:

"They then sent over an offer to pay them $7k (half up-front) for the preparation of an investment kit (incorporation, business plan, stock purchase agreement, etc.), and mentioned a 4% commission on money raised and 4-7% founders' share equity for them with a ~$3M series A open-ended round, and that we would be able to keep majority share for the founders at that valuation."

I don't know an investor that would be happy paying 4% management fees to a middle man, or one who would want to work through a middle man instead of directly with the team tasked with executing.

Perhaps I'm misunderstanding how you're presenting this. But if I am understanding correctly this sounds like a very bad situation to get mixed up in.

Don't walk away, run away.