I 2nd the idea of using an automotive motor, especially a windshield wiper motor, since they have worm gears that will hold a load without power, and are rated to run continuously. I see them priced around $15 on the surplus web sites.
In regards to the power rating, you can multiply the max current times the voltage to know its wattage capability. You can also convert the torque with the RPMs needed on your wench to determine wattage needed. Here is a web site that will do that, http://www.magtrol.com/support/motorpower_calc.html
For position sensing, there are several possibilities. But, I think the easiest to use is a multi-turn potentiometers. For example, $4.60 is this one, http://www.robotshop.com/en/dfrobot-rotation-sensor-v2.html
. It turns a maximum of 10-turns like most multi-turn pots. So, you have size your drum to keep it within 10 turns. If it needs more than 10 turns, you have to gear down your connection to the multi-turn pot.
Thinking about your torque needs, that decision will be driven by staying within the 10 turns because it will drive the decision about the drum diameter.
Note that a potentiometer is not the most durable device. So you don't generally want it on a robot wheel. But, you are not continuously moving it, and it should hold up for quite a while. I would make sure you have code to prevent it from oscillating if part of the pot starts to go bad and generates a varying signal. Just stop the motor after getting close to the target is enough.