Now that propeller may be 50% efficient so you will end up with 25W of thrust.
The thrust of the propeller is not measured as power, but as a force. The difference between force and power is important here. The force can be quite high, when the wind speed relative to the propeller is low (e.g. zero speed if the vehicle moves with the speed of the wind). A fixed efficiency number does not work here as this would lead to infinite force. On the other hand that same force when applied to the wheels that slow down the vehicle will create power proportional to the vehicle speed. With enough speed of the vehicle, which means enough speed of the wind, there can be more power available than needed (a constant power for a given force) to power the propeller.
The design calculation would be about like this: Chose a prop speed where is works good.
In standing air the prop needs a certain power to create enough thrust, let's say 2 x the force needed to overcome the vehicle friction. A constant friction force independent of speed is reasonable as there is no wind drag at the speed of the wind. So half that force would be available to work against the wheels and from that power drive the prop. The required wind speed for the vehicle to reach the speed of the wind would than be the required prop power divided by half the prop thrust. The quality of the prop (and the friction losses) determine the needed wind speed.
When driving at the speed of the wind, the prop sees standing air around it, and can thus produce the thrust (force) like standing in still air. The force is good to produce a power of wind speed times thrust. In the example half is needed to drive the prop and the rest is available power, e.g to accelerate or overcome friction. So you see, that it is possible to extract energy, even when going at the speed of the wind. It does not work with a simple sail, but it does with the prop to produce a force even with zero speed difference.
For the wind power available to a sail it would be proportional to (w-v)² for the force and w for the speed. So somehwat in between the formula given by electrodacus and IanB. The prop can improve on the factor (w-v)² and still produce some force at V=W. With the prop I am not sure the full area would be effectiv, but that would only be some costant factor, like a different sail shape that may also lead to a factor a bit off 0.5.