Re: used kestrel 2500 or 3500??
If youre looking to save some coin, look at the Brunton ADC Summit. You can get a new one cheaper than a used Kestral. They are more simple but read temp, pressure and wind. That IMHO is all you need for a LR shot. Humidity is small potatoes.
Long range shooting is a process that ends with a result. Once you start to focus on the result (how bad your last shot was, how big the group is going to be, what your buck will score, what your match score is, what place you are in...) then you loose the capacity to focus on the process.