What are my options for running an overpowered motor on this small vehicle I'm engineering?
Basically, you are designing your motor to overcome static friction from a standstill. Sand might provide a challenge since you need to worry about angled slopes etc.  

I'm going to approach this by providing an example. This should tell you how pick you motor correctly. However, you will have to adapt to your situation. For example, you said you wanted your robot to drive on sand? That will change the friction co-efficient.




If the robot is to move and over come static friction from a halt, the weight of the robot must be such that the maximum torque or stall torque is bigger than the force of friction. Below friction is proportional to the weight of the robot:



For this particular application Tstall = 85oz-in = 5.3125lb-in and the rwheel = 45mm = 1.77in. Therefore, Fstall = 3 lb. If the co-efficient of friction is &lt;= 1, then the robot has to be &lt; 3lbs in weight. A more liberal answer is obtained by multiplying by 2 since the robot has two motors => 6lbs. 

The motors specs are from this motor:

https://www.pololu.com/product/2286