I personally went for the external monitors to be used with my macbook pro as I do not like the glass screen of the iMacs for professional photo editing. It is fine and oh so pretty to look at but the reflections is bad for judging shadows and colours etc.
You would need to calibrate an iMac asap too if you were to buy one, I find them a bit over saturated from the factory.
Or if budget isnt that much of a problem, you can source an 8 core Mac Pro desktop that has been refurbished from the Apple store for around 3.5k and under. Or a new one etc.