- Horticulture - for growing
- Accounting - for running a business and keeping your books clean
- Marketing/Branding - to learn how to sell yourself/business
Those would be the three fields of study I'd most strongly consider. I'd hire a lawyer, since I wouldn't want to pursue a JD just to learn the nuances of the law. Plus, there are plenty of good lawyers who consult developing businesses in the legal/MMJ states and they have the rules memorized forwards & backwards. I'd consider that money well spent.
Horticulture would teach you how to run a big grow op, accounting will help you run the business and the marketing/branding would go a long way to helping you define yourself in an ever-saturated market. If it were me, I'd go for the first two (Horticulture and Accounting) and pursue marketing/branding education online or with books. If you could find a way to include some technical training as well, it'd come in very handy. Electrical would be at the top of the list.