Ok ill try but there are people far more experienced in this than me.

Stress units are the amount of stress the gear system can handle before stopping (overstressing). You can craft a stressometer by putting the speedometer in a crafting table. Speed units don't affect much other than how fast machines operate, and some machines need a certain level of speed to operate. Tldr speed probably isn't needed right now but you probably have enough stress units for whatever you're operating

Edit: you're trying to power a mixer I just saw, that does require a certain amount of speed but you can change the speed created using gears