I don't know what the conversion are between Liters and and Cubic Feet, so I have to make due here. Since you said your're diving dry, I assume the water is cold enought to warrent the use of an isolation valve. If you are relatively new to diving, I would recommend a single PST (or Faber) LP95 or LP104 with a right hand Sea Elite, Scuba Pro or Dive Rite 300 BAR H-valve. Get two Apeks non swival DS4(?) firsts with the TX50 second stage primary on a 5'-7' hose (depending on your ambitions). Get the unbalanced TX40 for the back up (26" hose). I'm not too familiar with the Apeks regs as I dive the near duplicate Zeagle line. Others can correct me if I have the numbers wrong.
Now you are set to get used to the DIR hose routing and have enough air to get some decent dives in. I'm not exactly sure which wing Halcyon would recommend for a single PST 104, but if doubles are definately in the future, get the 55lb Explorer and use it for both the single rig and the doubles. I use my 55lb wing for single AL 80's and double PST 95's, though I intend to purchase a 27lb wing some day.
If you're new, take a couple seasons to get used to it all. Once you're confident to get into doubles, you would be nearly half way there. You would need to buy the similar left hand valve and isolator, 3" Highland Mills SS bands, and another similar tank.
As far as the backplate is concerned, it depends on your particular buoyancy. I use the SS plate for everything and still need weight. I'd get the SS plate for starters.
I don't know why the Abyss reg isn't used other than the Apeks reg has a tremendous track record for reliability. I'm not sure how the Apeks prices compare, but the Zeagle T50D was around $350US and it performs at least as well as other more expensive regs like Scuba Pro's, Mares, and Atomics. In short, there's no real good reason to buy anything else, I guess.
Mike