My advice: First of all forget about the macbook air, that 128G HD will fill up faster than you realize and you will not be able to upgrade it. I'm also in the position of upgrading my 2007 macbook and I'm looking at the entry level 13inch Macbook Pro, I cant afford the retina, if you can then go for it. [although the cheaper 13inch is due soon]
I'm not slagging the Air, they are great machines but after a lot of research, I found out that the although the screen has a slightly higher resolution than the 13inch pro, the pro has better color gamut. Add the fact that it's got a 500GB HD that is user upgradable [even to solid state] and a faster cpu, I feel it would be better suited for photography work. A workaround could be and air and an external HD tho?
With regards to the camera, If you upgrade, you imac should be fine. Like I stated, I have a 2007 macbook and It handles 5D mkii files quite well. Your I mac should be more powerful than my macbook.
As for what to upgrade first, I'm unsure. How big a problem is getting the pictures to your mac then emailing the finished photos to your clients? The errors on the camera are worrying, may be time to upgrade. I'v taken some great gig shots with my 40d, you just have to hope the venue has powerful spotlights and wait for those to illuminate the subjects. But in situations where there is one or two weak lights then not even full frame will help. [Maybe 1dx tho