Get the COB200WD.
It is the one I use and it works great.
If you have to get a motor for it get a $12 closed endbell starterbox motor.
It will run on 3 volts and have enough torque and speed to cut anything.
The power base model looks great for $184. That is a good price.
I have found the Cobras to be the easiest to use due to how the handle on the cranks are designed.