Unless I have missunderstood your diagram, you need a high input impedance, not a low input impedance. Otherwise current flows into the buffer, and Im is no longer equal to IDUT. You also need a low output impedance to drive the guard.
Probably, you'll be best off buying something from Apex, such as the PA94 or PA95. They make opamps with rail-to-rail voltages of up to 2.5kV, and currents up to 50A (though not both at once).
These opamps are expensive. It is possible to avoid shelling out for a high voltage opamp, using transitors and a low voltage opamp. It's known as "bootstrapping" and it's probably much more effort than it's worth unless you are building hundreds or thousands of units.
You will of course have to check that the input offset, input impedance, output impedance etc. are appropriate. We can't help you with that unless you give some numbers on your requirements.
WhatRoughBeast's suggestion would work out cheaper, but would require a little more setting up. If this is a one-off, then it'll be quicker to just buy the opamp. I'd also be quite cautious about introducing a switching AC/DC or DC/DC supply in close proximity to what looks like a piece of precision measurement equipment - if you go down that route than a linear AC/DC converter might save some time.