Very broad question indeed. You'll get more success if you provide couple of models that you're interested in. And also let us know what type of diving you're doing. I'm a recreational diver so my biases are from that view point.
For mask, fins, and snorkels, a lot is personal preference. You will want to try before you buy, especially with fins.
Snorkel: Agree with Rich, you want a bottom purge. Makes life a lot easier. However, I do want to point out that after OW, many people do not carry a snorkel. But if you want one on you, you should consider stowable/foldable models. For example, look at the
Apollo Prestige.
Mask: I prefer low volume, with as much field of vision as possible. I use the Tusa Viewtrek and it works well for me. Tried a variety of masks, and this one fits the best of those that were low volume with large fields of vision. Another popular low volume/big field of vision mask is the Cressi Big Eye. Frameless masks like the Oceanic Shadow are popular as well. But haven't used it so personally can't comment. While a clear mask does not look as slick as a black mask and black skirt, a clear mask does let in more ambient light.
As for mask fit, I was taught/shown a different method for fit. Look up on at the ceiling and put the mask on. Have another person check for gaps and how well the skirt naturally fits your face. I have found that breathing or pushing in on the mask can create an artificial seal if you're too aggressive. Trying the mask with the snorkel on is a good suggestion. A leaky and uncomfortable mask will quickly ruin a dive.
Fins: I would rent these until you find one you like. A good LDS will let you demo fins. And when you do try them out, dive with them. Surface swim demo is different than being underwater. Certain fins are optimized for certain types of diving and kicking style. Here's the latest thread on
Split vs. Regular type of fins. On this board, you'll see Jet Fins recommended a lot. I dive the Mares Plana Avanti, and it's a good inexpensive fin. Make sure you try on the fin with your boot to insure proper fitting. As you have probably noticed, boots come in a variety of sizes and thickness of soles, and these affect how well it fits in the foot pocket.
As for Backplate/Wing (BP/W), it is a type of BC. The first type of BC that most recreational divers see in OW and as rental gear is a jacket style BC or stab(ilizer) jacket. Another type is the back inflate BC. Zeagle is a notable manufacturer of back inflate BCs. Another type is a hybrid of the back inflate and jacket. See the
Buoyancy Compensation Preference poll. Each type has it pros/cons and have their uses. Some divers are strict in their usage of a certain type of BC. Other divers change their BC based on the dives that they are doing. Here's the last of many threads comparing BC types:
Newbie question: Jacket vs Back Inflation 
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