Picture a lever attached to a mounted axis and this lever is parallel to the ground. If the lever is one inch long and you tie a 1 lb weight to the end, at the moment the lever begins to fall (t=0) you will be generating 1 in-lb of torque on the axis (assuming no friction in the system).
Duh, not very clear I'm sure...
Your 50 in*lb motor is another way of saying 50/12, or 4.16 ft*lb. I would think 4 ft lb to be enough to stir a mash, even one of your 25lb batches. You could test this by attaching a torque wrench to your mash stirring apparatus and operate it at the speed you want. The reading on the torque wrench is the required torque. This would be dependent of the thickness of the mash no doubt.
Now, a motor will produce that specific TORQUE at a specific (or range of) RPM. What speed do you need your mash to be stirred at?
Depending on the speed you need to stir at vs. the torque curve of the motor, you may need to employ mechanical gearing and/or speed control via PWM or simple current control.
Sounds like a cool project!