As a 3rd Year diver, I would say slow down and pump the breaks. At this point you don't know what you like. You should be able to get mask, boots, and fins for 100 to 200, depending on the quality. I believe 800 to get certified is high, I paid 175 each for myself, wife, and 2 daughters each through an independent instructor. We did have to pay another 200 or so total for a room and food at the only lake clear enough to dive in Oklahoma. All instruction was 1 instructor to the 4 of us. You might run a craigslist add or ask around this site as well.
I agree with leaving the spear gun alone for a bit. Work on buoyancy first and get that down pat before trying anything...including photography.
Once you are certified and convinced diving is for you. Rent, borrow, beg, multiple types and brands of BCs and buy the type you want. If "Basic" means basic training, you may end up somewhere with warm water, that makes a difference on your type of exposure protection. I'm good with a 3 mil full....in your neck of the woods....not so much.
I went nuts after my first breath underwater and started assembling 4 complete sets of equipment. The jacket type BCs for all.....now one daughter does not really like it, and one does not have time (and hates diving local). I upgraded my jacket type to a better one, then bought a Transpac, then found a great deal on a backplate/wing combo. I'd be $700 ahead if I would have just gone with the backplate or Transpac to begin with.
My point is, you don't know enough to know what is best for you right now. You would be money ahead renting or borrowing for a bit before you buy anything but basic equipment. By the way....diving in Cozumel is cheap if you can find cheap airfare.
Good luck and safe travels. If basic does mean military, thanks in advance for your service.
Jay