Picking Thermistor for Low Power Push-Pull Output Amplifier

This would probably work, but to be on the safe side, I'd use an optocoupler, or a similar electrical isolation device in place of using a single MOSFET. The problem is the GD capacitance of the MOSFET, which, if in a case of a high frequency gate voltage rise may result in a transient voltage rise on the microcontroller pin before this parasitic capacitor has a chance to charge up.

Using a series resistor to the gate will only make the gate capacitor charge slower, so I'd drop it completely. With a FET device, you almost never need a series resistor, since the gate is not composed of a diode which you need to protect from drawing too much current, as in a bipolar device. The size of the pullup resistor will dictate how fast the parasitic capacitance will charge, so choosing it to be as small as possible seems like a good option, but it'll increase the static power consumption of your circuit considerably.

Typically a thermistor is biased with a low constant current supply and the resulting voltage across it is measured to determine the temperature. If you select an op amp or instrumentation amp with sufficiently high input impedance (e.g. > 1 megohm) you can use this to measure the voltage across the thermistor without disturbing the incumbent ECU electronics. Due to the high input impedance, you should consider incorporating a low pass filter to reduce noise pick up.

The new op amp circuit can then drive an analog voltmeter that can be calibrated with a temperature scale. You can determine the op amp gain, offset, and scaling factor empirically if you do not have access to the NTC specs and knowledge of its bias current.