The most important element in the rig is the coupler. You have to get the standarized coupler for precision, and then next is the microphone. I'm not too experience in microphones, but some microphones are better than others, particularly in regards to noise (supposedly gets really costly for those with lowest noise input, if you are doing THD measurements).
GRAS couplers are industry standard, and I'm using IEC 60318-4 (60711) compliant coupler. Something like this
This coupler simulates the human ear. You place the iem in the aperture of this coupler, and on the other end the microphone to measure the sound output. Microphone will need a pre-amp, with maybe a Phantom Power supply (that comes with those cheap sound mixing interfaces). For the iem, you'd plug to a normal sound output device connected to the computer (ie. usb DAC/Amp device, etc.. or optical connection), so that you can set them as output and input device on the measurement software (I use REW).
There's some details in regards to mic calibration and settings you'd have to prepare so that everything is set-up for most accurate measurements. That is the most challenging part, but other than that, it's really easy to figure out. We're all audiophiles, we play with connections all the times.
Here's the thread on measurements.
https://www.head-fi.org/threads/general-iem-measurements-discussions.903455/