Finding the right motor
Things to ask yourself:

What is my input voltage?  AC or DC?  Are you running this off of a battery or line voltage or a control or something else?  

What am I doing with the motor?  Is it for a fan and will be running at one speed for 8 hours a day?  Is it for a servo type application?  What's the duty cycle?

What is the environment it will be running in?  Does it need to be dust proof?  Water-proof?  Explosion-proof?  Will the ambient temperature be 20 C, 65 C or -40 C?  

How many hours does it need to run before failure?  5000 hours, 20,000 hours, 50,000 hours?  

Your answers to those questions would give me a pretty good idea of the type of motor you need and what its rating should be.  

EDIT:  In general, sizing a motor is as easy as you suspect.  If you know the power you need the motor to output, then you find a motor that is rated for that power.  The rated power on the nameplate of a motor is the power that motor can continuously output without overheating.  Generally these ratings tests assume that the ambient temperature of the environment is below 40 degrees Celcius.  So if you have a motor that is going to be running at a constant torque and speed at a temperature below 40 C, finding a motor is simple.  Things get trickier when you have short duty cycles or a duty cycle where the torque changes abruptly or higher ambient temperatures.