If you are renting tanks in the keys, please buy a regulator with a yoke fitting, not DIN. HOG regs come DIN only, and there's an adapter. The adapter puts the reg much closer to your head, which for most people is very uncomfortable.
With the diving that you describe, buying a sealed first stage is NOT necessary or even advantageous. They are made specifically for cold and/or dirty water, which is pretty much the exact opposite of the FL Keys. For your diving any decent quality regulator will work absolutely fine. What's important is that your regs can be serviced competently in a convenient location. The zeagle envoy is a perennial 'best buy' and since zeagle is local, one would hope there's someone in your area that can service them.
Other excellent low-cost-but-high-quality regs would be the aqualung titan, scubapro MK2 or MK11, with either the C200 2nd stage or R380, or any of the sherwood regs EXCEPT the SR1. The best deals are often on used regs, as long as you can find someone to evaluate them and service them. Regulators are designed to be rebuilt, and the warranties often end up costing more than the reg is worth, so there's usually little risk involved in buying used regs, especially if you have a helper who can advise you on particular deals.
A BP/W set up is definitely good for novice divers. There are a million threads about it, so read away. Having a rigid plate couples the cylindrical tank to your flat back in a very efficient way, while the webbing harness is infinitely adjustable and out of your way while diving.
I agree that you will need some exposure protection; you'll never find yourself saying...."gee, I wish I weren't so warm and comfy doing all these dives." A 3mil suit would be fine, maybe just a dive skin in the summer. But you don't want to just dive in the summer, do you?
If your projected depth is truly 15-20 ft, I wouldn't bother with a computer, but they're so cheap that you might as well get one. Just get a simple wrist computer, oceanic and aeris make decent ones (lots of others do as well) and forget about the bells and whistles. What you need is depth/time, N2 loading (typically a bar graph of some sort), NDL, and an ascent rate indicator. They all have those features. Nitrox is a nice option, but really at that depth it's not worth spending a lot of money on, either for the class, the gas, or the computer. If you head into regular diving in the 60ft+ range, nitrox is much more appropriate.
Its a shame that these gear advice threads seem to be inching towards the "buy from me" vibe and away from the "here's what will work" vibe that is really central to forums' like these usefulness.
