Since the Analog Ins on the arduino mega are high impedance, do i still need a unity gain buffer for a simple thermistor (just a thermistor and resistor and measuring the divided voltage)?
No buffer needed as long as your resistor and thermistor don't go above 10 k?.
From the datasheet: "The ADC is optimized for analog signals with an output impedance of approximately 10 k? or less. If such a source is used, the sampling time will be negligible. If a source with higher imped- ance is used, the sampling time will depend on how long time the source needs to charge the S/H capacitor, with can vary widely. The user is recommended to only use low impedance sources with slowly varying signals, since this minimizes the required charge transfer to the S/H capacitor."
Even if it is more than 10k, you can still put a capacitor on that to mitigate it - as long as you don't need to measure fast changing signals.