After your mask, snorkel, and fins, consider purchasing a dive computer next. Dive computers are essential for dive safety. In addition, they are not usually part of diving rental gear. Usually, regulators are the last piece of gear purchased as they are an expense. However, if you are ready to invest in gear, some dive shops offer a package containing (BCD, Regulator, and Dive Computer).
See our detailed advice on when to buy diving equipment here.