I don't think a hs motor would necessarily be bad with hs gears, but I wouldn't do it. Especially running an M120, you will probably want a motor that has some good torque too, and hs motors don't have near as much torque. I am running a high torque set up in my M14 like I was saying earlier, and people were saying not to get infinite torque gears and motor, but instead to get double torque gears (there are also triple and infinite) and and infinite torque motor. That way you can either maximize your motor's potential, or your gear's potential.
And the Matrix is probably alright. Just know it is a clone company, and won't be completely top notch, I have heard some people say they like it though. Or you can just go with a TM EG1000, extremely reliable, and good balance or torque and speed. Search for some motor charts on ASR, they have stuff where they compare motors, and test their rof on different springs.
oh and don't forget you need a 9.6v battery, you got one of those?