I think 500 to 500 mesh powder should be fine with icecream.
As far as the hammer mill, find the one that has the largest screen as possible. It is critical to mix with lots of air during grinding process to keep the powder form getting hot and melt inside the mill. Perhaps mix the feed with dryice could keep it from melting during grinding.
The thermometer shown in the Amazon link seems to use 9V battery. Typically 9 V batteries are expensive compared to AA batteires. It may not be a big deal since the thermometer does not use much power. Something to consider for a long time use.
It is important to know how these thermometers work and what their limitations are. There are lots of information in internet. The temperature reading depends on how you point the thermometer, how far is the object form the thermometer and the angle etc. Of course, like others have mentioned, you have to keep stirring while you are making the measurement.
These need to be calibrated with ice or boing water or comparing with another thermometer that is NIST referenced. It may not be critical if you are not planning to follow other receipes that requires accurate temperature information. For day-to-day use where you are trying to repeat your own recipe developed uisng your thermometer, it may not be critical.
