John,
The more important danger zone to be aware of is the snake's strike range, as that is more limiting. The snake can detect prey before it can reach it (unless the prey animal sneaks up from behind
Anyway, to answer your questions, I've seen articles stating that they can sense less than half a degree of temp difference at distances over 2 meters.
I generally use, and would recommend, the 18 inch hemostats. There is a 24 inch version; but the ones I had didn't have "grooved" tips, so it was harder to grip the prey item (or whatever I happened to be grabbing)...though it isn't that hard to do a rough version of that at home. I've just found the 18 inch to be more functional over the years.