If you're going to use Xcode for development, then I suggest you think about your screen setup, perhaps more so than the exact specs.
For instance, if you use Interface Builder, you'll often want to use the assistant editor right next to it:
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Now, the above is a somewhat old picture; it's just an iPhone screen which is small. But nowadays you'll build universal apps and then there'll be an iPad-sized screen there.
A resolution of 2560x1440 is good enough to comfortably work with Interface Builder. I found the 1920x1080 resolution lacking and thus I wouldn't get the iMac.
You'll also probably start using Swift instead of Objective-C. Currently, the Swift compiler is pretty slow. It's getting faster all the time, but more than two cores helps a lot. But like the other poster said, get an SSD above that.
I'd get the middle model $699 Mac Mini and upgrade to a 256 GB SSD. Then with the budget left, I'd get a decent monitor, perhaps Dell Ultrasharp U2515H (which is 2560x1440 for around $330) or even better, Samsung U28D590D which is 3840x2160 for about $400. I'm just giving examples, I don't actually know if these are compatible or the best in their category.