Yes the SP MK 17 is a diaphram style regulator vs the piston style Mk 25 is is better suited for extreme colds. I believe the MK 25 is the highest flowing regulator on the market. Their Mk 17 isn't very far behind on the numbers! I have had my Mk 25 in the hi 40s and no problems. However, was in my pool @ 51 degrees and decided to purge some air from my tank.(just purchased the Mk 25 and had to play) wow that puppy flows!!!! A few seconds later I released the purge button, but it kept flowing, and flowing, and flowing. I quickly swam to the shallow end and stood up. I reached back and touched the first stage only to feel a giant ice block. The first stage froze solid! In only a couple of seconds it pretty much drained my 119 PST! Moral of the story is do not purge a butt load of air at depth in cold water with a SP Mk25.
Regarding the Air 2, I don't care for them. The buttons are very hard to push and honestly, I think they're a crappy design. Those regs take so many specialized tools to rebuild it aint funny! If you really like the idea of one less hose and want an Air 2 style backup, look at the Atomic SS1. The fit and finish is much nicer than the Scuba Pro, the buttons are easier to push, and they come in colors (dont forget about the titanium...)
Back plate: Ditto on everything everyone said. I prefer simple backplate $125, basic harness $25 with 2" crotch strap (I was very leary of ...discomfort, big waste of stress, totally cumfy) and a wing ~$300 I have a Dive Rite Rec wich is ~51lbs of lift.(depends where you read the specs...) I recently found out that this is enough lift to hold up my pair of steel 120s with all the little doo dads in fresh water so its more than enough for the sea. This makes me think I can get away with a smaller, more streamlined BC for when I dive my single tank (always except for my pool for now)
weights. lug around a weight belt, lug around weight pockets, or lug around loose weights that will cram into your optional quick release weight pockets. Personally I choose a weight belt. When Im humping all my gear to the dive boat, I can either throw the belt over a sholder, or just wear it like a belt. MOST dive bags are not designed to carry weight in them. So no matter what you choose, you gotta lug the lead anyway. Oh yeah a nice XS Scuba multi pocket weight belt costs a HELLUVA lot less then the optional quick release weight pockets for a backplate.