If you have any doubt when you join, don't take the gig. That doubt will only grow. Operational experience in building a company is huge. MBAs from top schools are good at putting together decks and analysis, but start up world is all about execution. Look for folks who could do your job.
Applied theory is worth a ton more than theoretical knowledge.
The ability to manage costs is huge. When in doubt, I think a general and easy to implement rule is to avoid consultants. Don't expect to get rich with it. Equity is a bonus if it works out. Make sure the salary is there.