If you have limited budget don't worry about retina. You're a developer, not a designer so that shouldn't be your priority. Think about getting some used macbook pro, replace the hdd with ssd for system and apps, and super drive with case for hdd and put there some larger hdd for data. The money that you save use to get more ram (and maybe faster ssd) and bigger display to fit simulators.

This setup should work for most cases giving you mobility, speed and space to work, when needed.