I am not sure I understand your goals. Are you looking for full time employment as a professional? Do you want that to be your primary source of income, or are you looking for part time money?
If you want to work full time, I would suggest that you aim for working for a successful dive operation and not as an independent business (unless you have access to enough investment capital to start your own business). By working for an established company, you can fill many roles as your skills and experience grow. You can be a DM for dives, instruct as needed, work the retail floor, clean the boats--whatever needs to be done. As you stay with the company, your skills and value grow. You could be adding skipper to your list of activities. Some companies will hire you in a minor role and then help you with the training as your role grows.
If you do that, you don't need to worry about business costs. The only costs that would impact you would be agency renewal and insurance costs, and the differences are not enough to make that the primary reason to choose one above the other. A number of years ago the Instructor forum on ScubaBoard had a thread in which a new instructor expressed his frustration because no one would hire him. How could he gain experience without working? It turns out he had chosen to be certified by an agency that would best be described as on the fringe. None of the shops in his area worked with that agency, so how could be seriously expect to be hired?
If you are instead looking to make part-time money as an instructor, you have more choices. South Florida is a good location for that because it is possible in many areas to be independent and do well. As was mentioned above, that means selecting an agency that allows you to work independently. If you do a good job for a while, it is possible to do enough business to make it your primary income.
