The MOSFET is a BS170 and the thermocouple is type K

The first thing you need to know is the output of the thermocouple. I am sure that you can find this for the temperature range in question. Data are avaliible from companies like Omega. Then you design an amplifier with the appropriate gain to give you the desired mv per degree. While you are working on this look up "instrumentation amplifier" which will give you a much more elegant solution.

BS170 - N-Channel Enhancement Mode Field Effect Transistor | Hqew.net