That was the reason it was suggested to use TL072 - it has a very high impedance so as to not cause interference on the circuit.
With respect to the capacitor - I'm not sure where I should pick up the input from the thermistor?
I can get it from before the resistors on the other side, or after it - in fact I could go as far back as the MCU that's reading it - is that the ideal place to measure from?
What will happen is I'll get the voltage reading and build an array of the voltage to temperature values and that should give me close enough to a reference of the temperature being read by the other controller..
What I think will work is:
Thermistor -> Existing controller.
Then solder to the existing input (either at the thermistor, before the resistors, or after the resistors, at the MCU), cable to bring it out to TL072 Op Amp following the voltage reading.
The TL072 as a voltage follower will feed the same voltage reading into my Arduino and it will reference it's table of voltages to decide what temperature it is.
So, do I get the TL072 input from the + of the thermistor (i.e. the field side of the thermistor), or from the MCU side which will be after the resistor (i.e. the midpoint)?