I've got a mid-2010 (holy crap it's getting old!) MBP 13" that I've used for iOS development for a long time. It's had a bit of an upgrade (1TB drive, 8GB ram) but nothing major. You can definitely do iOS development on something like this, and it's way underpowered compared to what you're looking at.

Having a second monitor (I've got a 24" Asus) at home/work is really handy, but I have no problem using just the main screen when I'm working mobile.