I went through something similar trying to sell a B2B PaaS company. A few comments:

1. Fees were structured as a small retainer up front and upon a transaction, either a % of the deal or a flat fee whichever was less. So the M&A firm was guaranteed a minimum amount for closing a deal. The retainer was about 20% of the minimum amount upon execution of a sale. The % fee for the M&A firm was tiered so that it dropped if the sale price was above a certain level.

2. Make sure you put in place a carve out for any potential acquirers you already have a relationship with or are talking to.

3. Watch closely to make sure they are able to get the right conversations with the right people quickly. It's a warning sign if they struggle to get calls returned.

4. Depending on how you structure it and how well plugged into the industry you are, you might consider working with a banker or getting advice from your existing "team" ( Board, investors, etc.) and just letting them advise you vice paying an M&A firm. This is sort of related to my earlier point in watching out for whether you are already talking to the right acquirers yourself (you said there has already been interest, so perhaps this is the case).

5. Assuming you are the CEO, plan to invest a lot of time in this yourself no matter who you hire or how you structure it. IOW you'll spend less time & attention running the business while you run this process instead. Make sure you are OK with this, and that your Board is as well.

Good luck, have fun, and let me know if you'd like to talk further.