what the hey... Ill throw in a couple pennies....
you are scheduled for GUE-F... you WILL be doing nitrox. Nitrox is now part of Fundimentals.
Nitrox ALMOST means nothing when it comes to buying regs anymore. MOST dive shops blend the mix not Partial Pressure...
Swivel vs non Swivel. I have Apeks Ds4s I also have Mk25. (MK25 is swivel DS4 is not) Yeah the MK25 routes hoses slightly better for doubles than the DS4, but it has nothing to do with the swivel, just the port on the bottom. I personally see no use for a swivel reg, Im not "It is a failure point and you will die!!@" But I see it as more parts, more machining, more money for nothing. By the way, its a failure point and you will die!!!!!
You say <$600 is that for everything or just the regs? Dont forget you will want your backup 2nd stage, along with your brass and glass SPG... Super duper shoppers can find the spg for ~$60, but they go for about 100 in most LDS, and a 2nd stage....another bill or two.
regarding the annual serive, Im not sure where in Cali you live, but Im just north of LA and the going price here is $30 a stage. So figure 90 a year plus parts. SP should be under warante, Apeks I think are free parts also (I dunno I have the parts and do it myself) if the Apeks aren't free, watch out as the part kits are $$$ I don't know what retail is on the parts, but cost is I think 15 for second stage and 22 for first. I don't have my Atomics price list with me, but I want to say they are about 15 for the kits also... Shoot me a PM, if we are local, I have a "classic downstream" (read non balanced 1st or 2nd) a MK25/S600, DS4/XTX40, TX 50. and ATX 50 you can try, I also have a MK15 D400. Ill let you try any that you want, I can show you diagrams how they work and let you make an educated decision.