I'm a big fan of both Deep6 and Atomic. I don't think the OP can really go wrong with either. It does sound like the Deep6 Signature regs might be the best choice for her, given her plan to get certified to work on them herself.
However, I do have a couple of thoughts about it all and want to clear up what I do think is a slight bit of misinformation.
- The Atomic Z2 regs will be just fine in 40 degree water. There is no real reason I can see for the OP to spend extra for M1 regs. 40 degrees is not that cold when it comes to regulator performance. At least, not when you're talking about quality regs like Deep6 and Atomic.
- If you want a swivel on a 2nd stage reg, I would highly recommend the 360 degree swivel that Cave Adventurers sells for $25 over an Omni-swivel or the Atomic Comfort Swivel. Those latter are just way over-priced - in MY opinion, anyway. I have the CA swivel on all my regs, including my Atomic Z2 set.
- The idea of self-servicing regs may be a bit over-hyped. I am a certified ScubaPro Pro Regulator Tech. I have some ScubaPro regs among my collection. I have never serviced them myself and probably will not. There are two main reasons for that. One, to be able to service them myself would require investment in tools that cost more than I really want to spend. And, two, I would only be servicing them every couple of years at the most. And, at that point, I would not want to depend on my own memory to do the service correctly. If I was working on regs somewhat regularly and often, then yes, I would service my own. But, for something I will only ever do every 2 (or longer, most likely) years, I will pay a professional who has all the right tools and does it every day.
So, you say you will get certified to work on your own, but will you really do that? Only you can decide that for yourself. For me, I will keep my regs properly maintained, so that I am extremely unlikely to actually have a problem in the field. And, instead of buying all the tools required, I will just take an extra 1st and 2nd stage with me on trips, so that if I do have a problem, I can just swap a reg out and then have the problematic one serviced when I get home.
- The Atomic Z2 regs are only a little bit more expensive than the Deep6 Signature regs. The price is close enough that I would not expect the difference to be the determining factor in the purchase. What should be noted and considered is that the Deep6 regs have a bottom port and swivel turret. The Z2 regs also have a bottom port but the turret is not a swivel. However, to ME, a swivel turret has no use in a single tank setup (but a bottom port IS, or can be anyway, useful, even in single tank). Thus why I have a Z2 setup for my single tank reg set. However, for regs for backmount doubles or sidemount or deco/stage/pony bottles, I DO want a swivel turret. So, what to buy would depend on what you think you might want to do in the future. Personally, I have found that switching a reg set back and forth between single tank and doubles and sidemount is a pain, so I have a dedicated set for doubles and a dedicated set for single tank. And other regs dedicated for deco bottle use. The concept of buying only regs that would work for all those scenarios sounded good to me, on paper. But, in reality, saving some money and getting Z2 regs for single tank meant not having a swivel turret - and that's just fine for me. I didn't spend extra for a feature that I have learned I will never need or use.