A cordless drill is not going to last long mixing mortar. It's a matter of horsepower and temperature more than torque. Torque measures how much leverage the drill can apply to a screw. Mixing is a continuous high load over a fairly long time and will burn out the winding or brushes. Although most cordless drills are brushless nowadays. If you're going to do a lot of mixing, go to harbor freight and buy a cheap 1/2" corded drill. Because even a 3/8" corded is insufficient for heavy mixing jobs.