How closely do the input buffer op-amps need to be matched on discrete instrumentation amplifiers?
At DC it doesn't much matter.
For good AC common-mode rejection you would be best to use two amplifiers on the same die at the inputs, otherwise you'll likely have a region where CMRR deteriorates. If you low pass filter the inputs such that those frequencies never make it to the inputs it may not matter much.



simulate this circuit &ndash; Schematic created using CircuitLab
If you do an AC simulation of the above, and there is a mismatch of 16MHz to 15MHz GBW in OA1 vs. OA2, the CMRR is more like -20dB than -60dB, which is pretty material.