around 35 mph with something smooth and light and hopefully not have to worry about cogging. i mainly just putt around and hope that this would be stronger than a mm7700 which is what i am using now. i just hope that it would have some nice grunt but not insane speeds.
Direct soldering is a good idea, it eliminates the connectors, which can cause problems.
If you have a motor that is undersized for the weight of the truck it will have to work too hard and will get quite hot. Generally a larger motor will stay cooler longer.