Rent the stuff that is easy to rent, buy the stuff that needs to fit right or is unavailable. If you dive cold (and you should!) buy a drysuit right now. Rentals never fit, always leak, and when they say "elastic band around the neck" just run away! Then get a flashlight; can't rent them and will enhance your enjoyment of all but the very clearest waters. Think 'magnetic switch', 'LED', and 'regular batteries'. Computer next; if you can rent it at all it will have a weird interface and be very conservative. Cheap one for now; if you upgrade later keeping the old one as backup can save your trip. Having my own pony with my own air in it makes me feel much better about sketchy rental regs, and is a good idea anyway. BCD, wetsuit, regs, tanks, and weights are all nice to own but they all work the same and every shop has them.