Welcome to the board and to diving. Typically, the Keys ops do not supply a DM and some require one if you are new to diving or have not dove in a long time. It's not a bad idea to hire one or find an experienced diver to go along with you for a few dives to get your feet wet, even if they are not required...sorry bad pun.
Renting gear is a good option for a while but if you decide you really like diving, buying your own is a good idea. While most rental gear is safe, it's rarely the best performing gear on the market. If most of your diving will be shallow reefs for the near future, I would make computers your last purchase. They are not necessary for that type of dives, you are better served by spending the money diving and gaining experience in the water.
Renting gear is a good option for a while but if you decide you really like diving, buying your own is a good idea. While most rental gear is safe, it's rarely the best performing gear on the market. If most of your diving will be shallow reefs for the near future, I would make computers your last purchase. They are not necessary for that type of dives, you are better served by spending the money diving and gaining experience in the water.