The 3 Pro is no longer available except used, it has a much more robust radio link [= longer range] than the P3 you can still get new as a basic model.
I wouldn't get one to check cattle exactly, but it can be useful to locate a group or individual as they are easier to spot from above, and you can cover a wide area in minutes.
If you get close you can spook them, the noise is a bit too close to swarms of bees I think.
Most farms will be able to partially justify one for a variety of reasons. Apart from being kind of fun obviously.