I recommend targeting the brand owners/marketing departments directly. You then can likely find out the decision makers and can make your pitch directly to them. I have found this easier than going through ad agencies.
I think you should give equity to anyone who's going to have an impact on your team. That's true for engineers, designers and especially marketing people. Some of the most important people are marketers - however, the best marketers are more product marketers then just a PR / community person. ...
Define the point where you can take a breather. Map the route between this here now and that there then. If you can count the steps along the way with confidence, that's good. If you know how you might change direction midway through or even turn back, that's good. If you can decide in advance ...