Going used can be a good way to go, but can depend a lot on luck. Check the link in my sig for information on my Nexus hub equip commuter. I love it. I bought a 1984 Trek 720 and put the Nexus 8 hub on it. I have only had 1 flat so far and did not think it was much different from a derailer bike as far as changing the flat. I commute on mine 4 to 5 days a week. 11-12 miles one way.

Having done the commute on both my bikes, one has the nexus hub, the other is a standard 9 speed rear with a triple up front, I can honestly say there is no real advantage or disadvantage of a nexus hub in Arizona. It is neat and low maintenance, but it is also heavier. The derailer requires a bit more maintenance, but it is easy to do, light weight and common parts. I don't think you would be going wrong with either. Like many things it is about preference.

I also find that it rains so little in Phoenix and Tucson, that fenders are not absolutely needed. If you are planning on raiding even when it does rain, then get some fenders. I do plan on getting fenders for my commuter but just have not done so yet (after 3 years of commuting)

I recommend you try to find a good local bike shop. If you are in or near Mesa, Paragon Cycles is Excellent. Ray (the owner) is a long time rider and commuter and can help.