I am using an agitator driven by a 240V, 60 RPM gear motor. Initially I wasn't going to use one because during the heating phase convection currents did a good enough job of keeping the water moving. However once the HLT reaches target temp, the water tends to stratify. Just by dipping my lab thermometer in I was seeing 10 degree changes in temperature.
I have a writeup on my HLT, the section on the agitator is towards the bottom.