While its very true grit can get in a treadmill motor and can be a problem but many have successfully have used them for years with a dust shield built protecting the motor while still being able to cool it. It can be a very cheap solution if you know how to cobble one together.
Please correct me if I'm wrong but a DC treadmill motor can deliver high torque at lower speed speeds with a PWM DC controller that has a constant torque operation feature on it giving it the rated torque at the rated armature current independent of speed allowing a constant torque capacity regardless of speed.
VFD are also excellent, especially since there prices have dropped so much recently and the availability of used & cheap 3 phase motors.
My apologies to the OP as we've digressed and gone off topic.