As for the monitor - I'm sure the BenQ is an excellent monitor, but what sets Eizo apart is their Colornavigator calibration software.
The most useful feature is that you can have as many different calibration targets as you like, each tuned to a specific paper or output condition. These can be switched with a single click, and CN not only changes the calibration parameters on the fly, but also loads the corresponding monitor profile on system level. All you need to remember is to relaunch PS/Lr etc so that the application can pick up the new profile at startup.
This way you are ensured a close to perfect match from screen to print under any circumstance.
No opinion on iMac vs. MBP. They should probably perform about the same.
Do Eizo and BenQ monitor have their own graphic card built into them?
They both have internal programmable LUTs - whether that qualifies as graphics card is probably semantics. In the Eizos it works in 16 bit depth, in the BenQ 14 bit. But this means calibration is performed internally in the monitor and not the video card. The point is that corrections can be made without the risk of banding and artifacts, which you frequently see with standard video card calibration. You also have vastly more precise control over the calibration parameters (white and black point).
The monitor profile, used by color managed applications only, is of course a different matter.