Just for the OP's sake. Bp/w stands for Backplate/Wing, and will be your most logical/economic choice if you intend to go into cave diving.
Bp/W's are made up of several components that together creat a buoyancy compensator.
You have your
- Backplate
made of Stainless Steel (SS), Aluminum (Al), or a synthetic material. It can be weighted to take ditchable weight off your weightbelt.
SS is usually 6lbs, Al 2lbs, synthetic 0ish-1lbs
- Wing
There are doubles and single dedicated wings. You should not aim to get wing that can "do" both. Too big a wing and it can cause stability and air dumping issues when using single tanks. Too small and it can cause problems with reaching your dump valve when diving doubles.
Doubles and singles require different lift characteristics, so you're looking at buying 2 different wings anyhow.
- Harness
There are the simplest single piece hogarthian harnesses and then there's the more complicated multiple piece harnesses that have quick releases.
- STA + cambands(tank straps)
STA's are single tank adapters that can take cambands. When diving doubles you won't be needing these as doubles bolt directly to your plate.
Recently some manufacturers are making wings that don't require STA's to stabilize tanks onto your Bp. In this case all you need then are the cambands.
TSandM, I found the adaptability of bp/w is almost true, but doesn't always work out in reality. Many bp/w divers I saw, they use SS plate for single wing, but AL for double, so more like another plate and another wing.
Have that said, I do agree starting with bp/w single setup, get used to it make the transistion to double much easier, even if you have to change plate and wing later
Grab an Al plate with a STA+6lb weighted insert and you've solved that problem.
Halcyon and Oxycheq make such an STA.
DSS (Deep Sea Supply) also makes bolt on weighted plates which has the same effect, except you don't have to futz around with an STA.