Horsepower achieved with multiple motors Scenario
I'm planning to build an electric motor for fun (not for any practical purpose). I recently purchased a shapeoko CNC mill kit and intend to manufacture most of the parts housing, stator etc. I'm currently in the planning stage and would like to plan/build this intelligently vs trial and error.
Goals
move 200lb (including the motors) at about 5-10mph (faster end of that if possible)
Questions


*

*How do you calculate the torque/hp that a motor will produce?

*If I put 4 smaller motors (instead of 1 powering the entire drive train) on the job can each produce less torque/hp and still achieve my goals?


I understand the basics of building an electric brushless motor, I'm more interested on how to plan a motor for a specific output.
EDIT
After some more research it seems that 1 HP = 745.699872 watts

so to move 200lb at 5 mph


*

*I need a 0.0019511579888525 horsepower motor.

*I need to provide 1.4549782617235 watts to the motor.


AND to move 200lb at 10mph


*

*I need a 0.01560926391082 horsepower motor.

*I need to provide 11.639826093788 watts to the motor.


I still need to figure out how Question 2 factors into my calculations.
 A: If the weight is $F$ in pounds, the coefficient of friction $\mu$ and the speed of $v$ in mph then the power $W$ required to maintain this motion in Watts is
$$ W  \approx 2.0 \mu \cdot F \cdot v $$ 
The coefficient of $2.0$ comes from the conversion into metric units. To move 200 lbs at 10 mph with a coefficient of friction of $\mu=0.4$ is
$$ W \approx 2.0 (0.4) (200\; {\rm lbf}) (10\; {\rm mph}) = 1600 \; {\rm Watt} = 2.15 \;{\rm hp} $$
This power comes from $W=I\,V$ where $I$ is current and $V$ is voltage, without losses or a fraction of this product with losses. I guess you can get the details from wikipedia. An online calculator is also here.
A: 
Goals move 200lb at 10mph.

The power required to keep a speed depends on the coefficient of friction and coefficient of drag. Both heavily depends on the shape of the object, the terrain on which the object moves, the types of tires used, the transmission used and so on. The question is not well posed.
But as an example the typical 250W e-bike usually reach about 25KM/h with the weight and the drag of a person. Most of the energy will be lost to air resistance in this case. The 1600W from the other answer seems a bit too much. 

If I put 4 smaller motors (instead of 1 powering the entire drive
  train) on the job can each produce less torque/hp and still achieve my
  goals?

In general, yes. The motors should produce 1/4 of the torque ad consequently 1/4 the the power in the same speed range.
