Op Amp for DC Voltage Measurement
The op-amp is non-critical- it just needs to be able to drive a few mA through the LED, and it needs to be "single supply" (input common-mode range includes ground) unless you have bipolar supplies. And reasonable bandwidth. 

IL300 does not make a particularly good isolation amplifier, the gain mismatch is 0.56 to 1.65 so it definitely requires calibration (your 800V could read as 448V or 1320V or anywhere in between,  plus resistor tolerances), and it can shift several percent for temperature changes. 

You might want to consider buying a commercial isolation amplifier or digitizing the signal and passing the value over the isolation barrier in digital form.