I've been using a 2011 11" macbook air for iOS development for the past few years. It is the bare-bones 2011 Core i3 model with 2 GB of ram and a 60 GB solid state drive. It is still as fast as the day I bought it (SSD and a lack of antivirus software really help!). It blows my "faster" windows laptops out of the water in everyday use.
My only problem is the drive space. I run Adobe Fireworks, Text Wrangler, xcode, Dropbox, and that's it. Every time I update xcode or the O/S (currently running on Mavericks) I have to delete a bunch of stuff then put it back on again. I am constantly bumping up against the 60 GB limit.
No matter what you buy if you buy new you will be able to run everything just fine. A new machine with 4 GB of ram works great. 8 GB is even better. Just make sure you get enough drive space to put your stuff on. 128 GB is probably fine, while 256 GB will give you plenty of room to grow for a long time.